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CAROLINA GEOLOGICAL SOCIETY1996 OFFICERSPresident: Ralph WilloughbyVice-President: Charles GardnerSecretary-Treasurer: Duncan HeronBoard Members:Alan DennisGeoff FeissCharles GardnerDuncan HeronRalph WilloughbyALPHABETICAL LISTING OF FIELDTRIP LEADERSAND AUTHORS OF GUIDEBOOK PAPERSWilliam J. ClearyOrrin H. PilkeyDepartment of Earth SciencesDepartment of GeologyUniversity of North Carolina at Wilmington Duke UniversityWilmington, NC 28403-3297 Durham, NC 2770-0228William A. DennisStanley R. RiggsUnited States Army Corps of EngineersDepartment of GeologyWilmington DistrictEast Carolina UniversityWilmington, NC 28403 Greenville, NC 27858James A. DockalStephen W. SnyderDepartment of Earth SciencesDepartment of MEASUniversity of North Carolina at Wilmington NC State UniversityWilmington, NC 28403-3297 Raleigh, NC 27695W. Burleigh Harris E. Robert ThielerDepartment of Earth SciencesDepartment of GeologyUniversity of North Carolina at Wilmington Duke UniversityWilmington, NC 28403-3297 Durham, NC 2 7708-0228Lynn A. LeonardHugo ValverdeDepartment of Earth SciencesDepartment of GeologyUniversity of North Carolina at Wilmington Duke UniversityWilmington, NC 28403-3297 Durham, NC 27708-0228ii

TABLE OF CONTENTSForward and Acknowledgements...................................................................................................... ............. ivOverview of the Marine Paleogene, Neogene and Pleistocene DepositsBetween Cape Fear and Cape Lookout, North Carolina ............................................................... ............. 1W. Burleigh HarrisThe Coquinas of the Neuse Formation, New Hanover County, North Carolina........................... ............. 9James A. DockalShoreface Processes in Onlsow Bay................................................................................................... ............. 19E. Robert ThielerMorphology and Dynamics of Barrier and Headland Shorefacesin Onlsow Bay, North Carolina.......................................................................................................... ............. 29Stanley R. Riggs, William J. Cleary, and Stephen W. SnyderInlet Induced Shoreline Changes: Cape Lookout - Cape Fear....................................................... ............. 41William J. ClearySedimentology and Depositional Processes in the Tidal Marshesof Southeastern North Carolina......................................................................................................... ............. 51Lynn A. LeonardShoreline Stabilization in Onslow Bay.............................................................................................. ............. 59Hugo Valverde and Orrin H. PilkeyFort Fisher Revetment Project........................................................................................................... ............. 65William A. DennisEnvironmental Coastal Geology:Cape Lookout to Cape Fear, NC (Fieldtrip Guidebook)................................................................. ............. 73William J. Cleary and Orrin H. PilkeyAppendices........................................................................................................................................... ............. 109iii

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 1-8AN OVERVIEW OF THE MARINE TERTIARY AND QUATERNARY DEPOSITS BETWEEN CAPE FEAR ANDCAPE LOOKOUT, NORTH CAROLINAW. Burleigh HarrisDepartment of Earth SciencesUniversity of North Carolina at WilmingtonWilmington, NC 28403-3297ABSTRACTTertiary and Quaternary marine sediments in the NorthCarolina Coastal Plain are distributed on two crustal blocks,the Onslow and the Albemarle. Differential uplift and subsidenceof these blocks about the Neuse Hinge has controlledpatterns of relative coastal onlap, sediment distribution, andthe positions of late Tertiary and Quaternary scarps and terraces.Paleocene sediments (Beaufort Group) are restrictedprincipally to the southern part of the Onslow Block andnorth of the Neuse Hinge. Eocene sediments (Castle HayneLimestone and New Bern Formation) represent the mostwidely distributed early Tertiary unit in North Carolina, andare found over most of the Onslow Block. Oligocene sediments(Trent, Belgrade and Silverdale Formations) arelocally distributed north of the New River, north of theNeuse Hinge and in Onslow Bay. The Miocene Pungo RiverFormation is mainly developed north and east of the WhiteOak River and Neuse Hinge, respectively. The PlioceneDuplin Formation represents the most extensive marineonlap during the late Tertiary, and occurs as outliers overmuch of the Onslow Block. The Pliocene Chowan River andBear Bluff Formations are developed north of the NeuseHinge and in the Cape Fear area, respectively. The lower andmiddle Pleistocene Waccamaw/James City Formations, andSocastee/Flanner Beach Formations are mainly developed onthe southern part of the Onslow Block and north of the WhiteOak River. ( Plio-Pleistocene distribution is related to formationand ~ development of the Hanover-Surry Scarp, Bogue-Suffolk Scarp and the Alligator Bay Scarp.INTRODUCTIONThis paper presents a summary of the Tertiary and Quaternarymarine stratigraphy and discusses the associatedscarps and terraces. In-depth discussions of North Carolinaearly Tertiary stratigraphy are presented by Harris and Zullo(1991), Harris et at. (1993), Harris and Laws (1994; inpress), Laws (1992), and Zullo and Harris (1987). Miocenestratigraphy for the area is discussed by Snyder et at. (1991 ).Pliocene and Pleistocene stratigraphy is discussed by Wardet at. (1991) and Soller and Mills (1991 ). Dockal (this volume)discusses upper Pleistocene units on the Cape FearArch.The area along the North Carolina-South Carolina stateline (approximate axis of the Cape Fear Arch) and the WhiteOak/Neuse Rivers (Neuse Hinge) is referred to as theOnslow Block (Harris and Laws, in press). To the north ofthe Neuse Hinge is the Albemarle Block. Differential upliftand subsidence of these blocks has controlled the stratalgeometries and patterns of relative coastal onlap on eachblock, the distribution of Coastal Plain units, and the positionsof late Tertiary and Quaternary scarps and associatedterraces.TERTIARYTertiary units on the Onslow Block are assigned to theTejas Megacycle of Haq et at. (1987), and are represented bythe Paleocene Beaufort Group, the Eocene Castle HayneLimestone and New Bern Formation, and the OligoceneTrent, Belgrade and Silverdale Formations [Fig. 1 ). In thispaper, the terminology of Baum et at. [1978) as modified byZullo and Harris (1987) and Harris and Laws (1994) is followed.PaleoceneThe Paleocene Beaufort Group represents two depositionalsequences (Fig. 1 ). The oldest sequence, the TA 1.2, isrepresented by the Yaupon Beach Formation of Danian age.The youngest sequence, TA2.1 , is represented by the BaldHead Shoals Formation of Thanetian age.Danian (Figure 2a)The Yaupon Beach Formation is recognized only on thesouthern part of the Onslow Block on the axis of the CapeFear Arch (Harris and Laws, 1994). It consists of olive greento gray, very fine to fine-grained slightly argillaceous bioturbatedquartz sand. A moderately to well preserved, lowdiversity nannofossil assemblage including lower Daniantaxa Cruciplacolithus prim us, C. tenuis, Ericsonia cava,Biscutum spp. and Neochiastozygus sp., and Cretaceous survivorspecies Placozygus sigmoides, Markalium in versusand Cyclogelosphaera reinhardtiiis present. This assemblage,in the absence of Chiasmolithus danicus, correlates tothe lower Danian Cruciplacolithus tenuis Zone (NP2 or CP1b).1

W. Burleigh HarrisFigure 1. Generalized early Tertiary lithostratigraphy andsequence stratigraphy on the Onslow Block, North Carolina,modified from Harris et al. (1993) and Harris and Laws (1994,in press).Thanetian (Figure 2b)The Bald Head Shoals Formation is also restricted to thesouthern part of the Onslow Block; however, Thanetian agedsediments are recognized at several localities in PenderCounty. The Bald Head Shoals Formation consists of almost7 m of sandy, molluscan-mold mudstone, wackestone topackstone, and contains very sparse calcareous nannofossilsand foraminifers, but abundant gastropods (turritelline) andpelecypods. Three mollusks that are age-diagnostic in theGulf Coastal Plain are identified; the gastropod Mesaliabiplicata Bowles, and the pelecypods Barbatia (Cucullaearca)cuculloides and Acanthocardia (Schedocardia) tuomeyi.Units that contain these mollusks in the Gulf CoastalPlain are considered Thanetian in age (Mancini and Tew,1988). The occurrence of the benthic foraminiferal speciesCibicides neelyi, Eponides lotus, Anomalinoides umboniferus,and Cibicidina sp. support a Thanetian age.EoceneFive Lutetian to Priabonian depositional sequencesoccur on the Onslow block from Brunswick to CarteretCounties. They are represented by the Castle Hayne Limestoneand the New Bern Formation (Fig. 1).Lutetian-Bartonian (Figure 3a)Three Lutetian and Bartonian depositional sequencesFigure 2. A. Isopach of Danian sediments on the OnslowBlock between the axis of the Cape Fear Arch and the NeuseHinge (modified from Harris and Laws, in press). B. Isopachof Thanetian sediments on the Onslow Block between the axisof Cape Fear Arch and the Neuse Hinge (modified from Harrisand Laws, in press).(TA3.3, TA3.4, and TA3.5/3.6) are recognized and assignedto the Castle Hayne Limestone. Where indurated, thesesequences consist dominantly of sandy, bryozoan and molluscanbiomicrudite and biosparrudite, and locally phosphatepebble conglomerate. Where unindurated, they consist ofbryozoan sand, which is often glauconitic.Priabonian (Figure 3b)Two younger sequences are recognized and assigned tothe Castle Hayne Limestone. The older (TA4.1 ) straddlesthe Bartonian- Priabonian boundary with the surface of maximumflooding approximating the stage boundary. Transgressivedeposits are interpreted below this surface to beLutetian/Bartonian; highstand deposits above this surface areinterpreted to be Priabonian in age. The high stand part ofthe sequence is the thickest and best developed, therefore, it2

OVERVIEW OF THE MARINE TERTIARY AND QUATERNARY DEPOSITS BETWEEN CAPE FEAR AND CAPE LOOKOUT, NCFigure 3. A. Isopach of Lutetian and Bartonian sediments onthe Onslow Block between the axis of the Cape Fear Arch andthe Neuse Hinge. Note that outliers occur on the Onslow Block(modified from Harris and Laws, in press). B. Isopach of Priaboniansediments on the Onslow Block between the axis of theCape Fear Arch and the Neuse Hinge (modified from Harrisand Laws, in press).is included in this section for discussion. The youngersequence (TA4.2 or TA4.3) is exclusively Priabonian in ageand has a more restricted spatial distribution. Both sequencesconsist predominantly of. bryozoan, sponge and molluscanbiomicrite and biomicrudite except along the northern part ofthe Onslow Block. Along the Neuse Hinge, the New BernFormation that represents either the TA4.2 or TA4.3sequence consists of sandy pelecypod-mold biosparite andbiosparrudite.Oligocene/Early MioceneFigure 4. A. Isopach of Reupelian sediments on the OnslowBlock between the axis of the Cape Fear Arch and the NeuseHinge (modified from Harris and Laws, in press). B. Isopach ofChattian sediments on the Onslow Block between the axis ofthe Cape Fear Arch and the Neuse Hinge (modified from Harrisand Laws, in press).One Rupelian, several? Chattian and one Aquitaniansequence are recognized between Brunswick and CarteretCounties (Fig. 1). The Rupelian sequence (TA4.4) is representedby the Trent Formation of Baum et al. (1978), theChattian sequences (TB1.1-1.4) by the Belgrade/SilverdaleFormations, and the Aquitanian part of sequence TB 1 .4 bythe Crassostrea channel deposits of Baum et al. (1978) andZullo and Harris (1987).Rupelian (Figure 4a)The Trent Formation is confined to the area between theNew and Neuse Rivers. In the vicinity of the Neuse Hinge itconsists of three ascending lithofacies; sandy echinoid biosparite,sandy pelecypod-mold biomicrudite and barnacle,pelecypod-mold biosparrudite. To the south near Jacksonvilleit consists of sandy foraminiferal silt and silty clay. Thissequence is assigned to the TA4.4 cycle based on the occurrenceof the barnacle Lophobalanus kellumi and the pectinid3

W. Burleigh HarrisChlamys trentensis (Zullo and Harris, 1987), mollusks thathave early Vicksburgian (Rupelian) affinities (Rossbach andCarter, 1991), foraminifers indicative of the Globergerinaampliapertura Zone (P19/20) (Zarra, 1989), and calcareousnannofossils indicative of zones NP21-22 (Worsley andTurco, 1979).Chattian-Aquitanian (Figure 4bThe Chattian Belgrade and Silverdale Formations arerestricted to quarries and core holes from about the NewRiver in Onslow County northward through eastern Jones,Craven and Carteret Counties. Chattian sequences are alsowell developed in Onslow Bay (Snyder et al., 1991). TheAquitanian Crassostrea channel deposits are only foundwithin a few kilometers north and south of the White OakRiver. The Belgrade Formation consists of about 8 m ofsandy, pelecypod-mold biomicrudite with minor interbeds ofquartz sand. The Silverdale Formation consists of about 3 mof mollusk- rich quartz sand, which is occasionally lithifiedand moldic. It occurs downdip (eastward) of the BelgradeFormation and is considered equivalent in age. Calcareousnannofossils (Laws and Worsley, 1986; Laws, 1992; Parkerand Laws, 1991), planktonic Foraminifera (Zarra, 1989), andmegafauna indicate that the Belgrade and Silverdale Formationsspan planktonic foraminiferal zones P21 and P22(Zullo and Harris, 1987). The Belgrade and Silverdale Formationswere suggested by Zullo and Harris (1987) to representfour depositional sequences ranging in age fromChattian to Aquitanian (TB1. 1-lower part of 1.4). TheAquitanian Crassostrea channel deposits were interpreted byZullo and Harris (1987) to represent the highstand of theTB1.4 sequence.Figure 5. Late Tertiary and Quaternary lithostratigraphy ofthe Onslow Block.foraminifers, calcareous nannofossils, diatoms and radiolarians,the three Miocene depositional sequences were dated bySnyder et al. (1991) as Burdigalian (Frying Pan Sequence),Langhian (Onslow Bay Sequence), and Serravallian (BogueBanks Sequence).MioceneMiocene sediments onlap the emerged Coastal Plainalong a north-south line that approximates the White OakRiver and are referred to the Pungo River Formation (Snyderet al., 1991) (Figs. 5 and 6). The Pungo River Formation isbest developed on the Albemarle Block and in Onslow Bay.Based on seismic analysis, Miocene sediments are interpretedto represent three unconformity bounded packagesidentified as the Frying Pan, Onslow Bay and Bogue BanksSequences. Lithofacies of the Frying Pan Sequence includemuddy, quartzitic phosphatic sand; organic-rich, phosphaticmud; and molluscan-barnacle shell gravels interbedded withquartz sand or foraminiferal quartz sand (Riggs and Mallette,1990). The Onslow Bay Sequence consists of calcareousmuds and biogenic sands and gravels with varying amountsof siliciclastic sand and chert (Riggs and Mallette, 1990).The Bogue Banks Sequence consists mainly of siliciclasticmuds and sands; the sands usually contain minor phosphateand the muds usually contain abundant silt-sized dolomite(Riggs and Mallette, 1990). Based on study of planktonicFigure 6. Distribution of Miocene sediments on the OnslowBlock (modified from Brown et al. 1974, and Snyder et al.,1991)PliocenePliocene units in North Carolina are referred to as theDuplin/Yorktown Formations and the Bear Bluff/ ChowanRiver Formations (Figs. 5, 7a and 7b). The Yorktown Formationis usually used for lower and lower upper Pliocene sedimentsthat occur north of the Neuse Hinge on the AlbemarleBlock (Ward et al., 1991). The Duplin Formation is used forage equivalent sediments that occur south of the NeuseHinge on the Onslow Block. The Chowan River Formation is4

OVERVIEW OF THE MARINE TERTIARY AND QUATERNARY DEPOSITS BETWEEN CAPE FEAR AND CAPE LOOKOUT, NCFigure 7. A. Distribution of the Pliocene Duplin Formation andequivalents (TB3.6 Sequence) on the Onslow Block. Sequencedesignation is after Zullo and Harris (1992). B. Distribution ofthe Pliocene Chowan River/Bear Bluff Formations (TB3.8Sequence) on the Onslow Block (modified from Ward et al.,1991). Sequence designation is after Zullo and Harris (1992).also used for latest Pliocene sediments that occur on theAlbemarle Block, and the Bear Bluff for age equivalent sedimentson the Onslow Block. The Duplin Formation consistsof sand, sandy and silty clay, and very shelly sand commonlyoverlying a basal phosphate pebble conglomerate. North ofthe Neuse Hinge the Rushmere and Mogarts Beach Membersof the Yorktown Formation (=Duplin Formation) are continuous;however, to south on the Onslow Block, the Duplin ispreserved as outliers. The thickest section of the Duplin Formationalso occurs to the south where almost 5 m are foundin Bladen County (Ward et al., 1991). However, most outlierson the Onslow Block contain less than 2 m of the DuplinFormation.The upper Pliocene Chowan River Formation is onlyused in North Carolina north of the Neuse Hinge; to thesouth, the Bear Bluff Formation of DuBar et al. (1974) isFigure 8. A. Distribution of lower Pleistocene Waccamaw/James City Formations (TB3.9 Sequence) on the Onslow Block(modified from Owens, 1989; Ward et al., 1991). Sequence designationis after Zullo and Harris (1992). B. Distribution of themiddle Pleistocene Socastee/Flanner Beach Formations on theOnslow Block (modified from Mixon and Pilkey, 1976; Owens,1989; Ward et al., 1991).recognized. The Bear Bluff Formation is known mainly fromthe area south of the Cape Fear River, and may occur adjacentto the Intracoastal Waterway below the Waccamaw Formationon the central Onslow Block. The Bear BluffFormation consists of calcareous sandstone, sandy limestone,subarkosic sand, and calcareous silt and has a maximumobserved thickness that exceeds 33 m (DuBar et al.,1974).QUATERNARYPleistocene {Figures 8a and 8b)Pleistocene geology along the seaward side of theOnslow Block south of the Neuse River is poorly known.5

W. Burleigh HarrisMixon and Pilkey (1976) mapped the geology of the CapeLookout area (Carteret-Craven Counties), Owens (1989)mapped the Florence, South Carolina, and North Carolina,10 x 20 Quadrangle (Brunswick and western New HanoverCounties), and Dockal (this volume) is currently examiningthe area of southern New Hanover and Brunswick Counties.However, no detailed geologic mapping of Pleistocene unitshas been completed on the Onslow Block south of NewRiver. The following discussion is mainly of those areasmarking the southern and northern parts of the OnslowBlock.The Waccamaw/James City Formations are used forearly Pleistocene sediments of similar lithology that occur onthe southern and northern parts of the Onslow Block, respectively(Figs. 5 and 8a). The Waccamaw Formation occursover most of the area south of the Cape Fear River, particularlyin low areas developed on older units (i.e., the PeedeeFormation), and north of the Cape Fear River in small pitsand dredge spoils just west of the Intracoastal Waterway. Ithas also been identified in Burnt Mill Creek in New HanoverCounty and probably occurs at other lower elevation localitiesthat are associated with the margins of the OnslowBlock. The Waccamaw Formation consists of poorly to moderatelywell sorted fossiliferous fine to coarse sand whichgrades upward into unfossiliferous sediments (Owens,1989). A local thin conglomerate of phosphate and quartzpebbles occurs at the base of the unit. Sediments that containtypical Waccamaw fossils range in thickness to almost 7 m(DuBar et al., 1974) in Brunswick County.The James City Formation of DuBar and Solliday(1963) is recognized along the Neuse River below NewBern. It extends to the north onto the Albemarle Blockalmost to Virginia, and to the south to the New River (Blackwelder,1981). Several small pits west of the IntracoastalWaterway between the New and Cape Fear Rivers indicatethat the unit extends to the southeast eventually becomingthe Waccamaw Formation (Ward et al., 1991). AlthoughWard et al. (1991) indicate that the Waccamaw/James CityFormations extend west of the Hanover Scarp, I know of nolower Pleistocene marine sediments on the central OnslowBlock north of the Cape Fear River. The James City Formationis an unconsolidated shelly argillaceous sand and sandyclay (DuBar and Solliday, 1963). Although the unit is consideredto be early Pleistocene in age (Ward et al., 1991),Campbell (1993) suggested that it is late Pliocene based onoxygen isotopes.Numerous lithostratigraphic names have been applied tomiddle and upper Pleistocene units between the Cape Fearand Neuse Rivers (Fig. 5). Middle Pleistocene units arereferred to the Socastee/ Canepatch and Flanner Beach Formations(Soller and Mills, 1991). The Socastee Formation,the major coastal Pleistocene unit in the Cape Fear region,consists of basal coarse sand, fine gravel and reworked shellsto 1 m in thickness, and interbedded sand and clay. The sandand clay are commonly peaty and contain upright tree trunks(Owens, 1989). The Socastee ranges up to 5 m in thickness(DuBar et al., 1974) in the northern coastal area of SouthCarolina. Its extent between the Cape Fear and Neuse Riversis unknown; however, if present, it is probably restricted tothe seaward edge of the Onslow Block (Fig. 8b). The CanepatchFormation, named for exposures in the Myrtle Beach,South Carolina area by DuBar (1971), was restricted to onesubsurface locality along the Intracoastal Waterway byOwens (1989). Therefore, the name is not used in this paper.The Socastee Formation is middle Pleistocene in age basedon isotopic dates (McCartan et al., 1982).In the Neuse River area, the Socastee Formation is correlatedto the Flanner Beach Formation of DuBar and Solliday(1963). The Flanner Beach Formation consists ofunconsolidated clay, sandy clay, argillaceous sand, and peatysand and clay, which reach almost 12 m in thickness; molluscanfossils are common in the lower part (DuBar and Solliday,1963). Although the Flanner Beach Formation wasrestricted to exclude some of the originally defined parts ofthe unit by Mixon and Pilkey (1976), the unit occurs seawardof north-trending elements of the Suffolk Scarp. The FlannerBeach Formation is also considered to be middle Pleistocenein age; its distribution is shown in Figure 8b.Late Pleistocene units are poorly described and areassigned numerous lithostratigraphic names. Dockal (thisvolume) discusses upper Pleistocene stratigraphy in the CapeFear region.SCARPS AND PLAINS ON THE ONSLOWBLOCKSeveral scarps and associated terraces (plains) are recognizedon the Onslow Block between Cape Fear and CapeFigure 9. Relation of scarps and terraces to major structuralfeatures associated with the Onslow Block (modified from Zulloand Harris, 1979; and Harris and Laws, in press).6

OVERVIEW OF THE MARINE TERTIARY AND QUATERNARY DEPOSITS BETWEEN CAPE FEAR AND CAPE LOOKOUT, NCLookout (Fig. 9). Zullo and Harris (1979) recognized threescarps that formed the seaward borders of tilted plains in thearea: the Hanover Scarp, the Bogue-Suffolk Scarp, and theAlligator Bay Scarp. The Hanover Scarp originated at aninterpreted cape in central New Hanover County north of theCape Fear River. To the south, Zullo and Harris (1979) suggestedthat the scarp paralleled the north side of the CapeFear River for several kilometers eventually becoming theSurry Scarp 80 km inland of the coastal margin. AlthoughFlint (1940) recognized the Surry Scarp inland on theOnslow Block, Zullo and Harris traced the Hanover Scarpnortheastward to just south of the New River where it turnedabruptly to the north eventually merging inland along theNeuse Hinge with the Surry Scarp. Soller and Mills {1991)followed the identification and location of the Surry Scarp asmapped by Flint {1940), and did not recognize the HanoverScarp. The plain delimited on the Onslow Block by theOrangeburg Scarp and the Hanover-Surry Scarp is identifiedas the Duplin Plain {Zullo and Harris, 1979). Sediments ofDuplin age represent the youngest marine formation underlyingthe area. Zullo and Harris {1979) indicated that DuplinPlain was at an elevation of more than 12 m in central NewHanover County and over a distance of 60 km graduallyincreased to about 21 m on the west side of the New River.The Bogue-Suffolk Scarp is located seaward of theHanover Scarp and essentially delimits the modern mainlandcoast on the Onslow Block. Mixon and Pilkey {1976)mapped the Bogue Scarp north of the New River, and indicatedthat in central Carteret County, it abruptly turned northand became part of elements associated with the SuffolkScarp. The plain delimited by the Hanover Scarp and theBogue-Suffolk Scarp is called the Waccamaw/CanepatchPlain {Zullo and Harris, 1979) and ranges in elevation fromabout 7.5 m in central New Hanover County to over 10.5 mjust north of the New River. Waccamaw and James City Formationsediments represent the youngest marine sedimentsunderlying the plain. Zullo and Harris {1979) also proposedthe Alligator Bay Scarp for a linear feature that occurred seawardof the Bogue Scarp between Spicer and Alligator Bays,Onslow County. The plain bounded by Bogue Scarp and theAlligator Bay Scarp rose from sea level 12 km south of NewRiver to about 4.5 mat New River and was designated theSocastee Plain. North of New River Inlet, the Alligator BayScarp may merge with the Bogue Scarp, forming the westernlimit of the Core Creek Sand.ACKNOWLEDGMENTSI thank the University of North Carolina at Wilmington,and the Center for Marine Science Research for providingpartial support for this work. Appreciation is also expressedto William J. Cleary for the invitation to submit a paper forthe guidebook, and James A. Dockal for his careful review ofthe paper. This is CMSR contribution #143.REFERENCESBaum, G.R., Harris, W. B., and Zullo, V.A., 1978, Stratigraphicrevision of exposed middle Eocene to lower Miocene formationsof North Carolina: Southeastern Geology, v. 20, p. 1-19.Blackwelder, B. W., 1981, Stratigraphy of the upper Pliocene andlower Pleistocene marine and estuarine deposits of northeasternNorth Carolina and Virginia: U.S. Geological Survey Bulletin1502-B, 16 p.Campbell, L. D., 1993, Pliocene mollusks from the Yorktown andChowan River Formations in Virginia: Virginia Division ofMineral Resources, Publication 127, 259 p.DuBar, J.R., 1971, Neogene stratigraphy of the lower Coastal Plainof the Carolinas: Atlantic Coastal Plain Association, 12thAnnual Field Conference, Myrtle Beach, SC, 128 p.DuBar, J.R., and Solliday, J.R., 1963, Stratigraphy of the Neogenedeposits, lower Neuse Estuary, North Carolina: SoutheasternGeology, v. 4, p. 213-233.DuBar, J.R., Johnson, H.S., Thorn, B., and Hatchell, W.O., 1974,Neogene stratigraphy and morphology, south flank of the CapeFear Arch, North and South Carolina; in R.Q. Oaks and J.R.DuBar, eds., Post-Miocene stratigraphy, central and southernAtlantic Coastal Plain: Utah State University Press, Logan,Utah, p. 139-173.Ferenczi, I., 1959, Structural control of the North Carolina CoastalPlain: Southeastern Geology, v. 1, p. 105-116.Flint, R.F., 1940, Pleistocene features of the Atlantic Coastal Plain:American Journal of Science, v. 238, p. 757-787.Gibson, T.C., 1983, Stratigraphy of Miocene through lower Pleistocenestrata of the United States Central Atlantic CoastalPlain; in C.E. Ray, ed., Geology and Paleontology of the LeeCreek Mine, North Carolina: Smithsonian Contribution to Paleobiology53, p. 35-80.Haq, B.U., Hardenbol, J., and Vail, P.R., 1987, Chronology of fluctuatingsea levels since the Triassic: Science, v. 235, p. 1156-1167.Harris, W.B. and Zullo, V.A., 1991, Eocene and Oligocene geologyof the outer Coastal Plain; in J.W. Horton and V.A. Zullo, eds.,The Geology of the Carolinas: University of Tennessee Press,Knoxville, Tennessee, p. 251-262.Harris, W.B. and Laws, R.A., 1994, Paleogene sediments on theaxis of the Cape Fear Arch, Long Bay, North Carolina: SoutheasternGeology, v. 34, p. 185-199.Harris, W.B. and Laws, R.A., in press, Paleogene Stratigraphy andsea-level history of the North Carolina Coastal Plain: Globalcoastal onlap and tectonics: Sedimentary Geology and Evolutionof the Atlantic Coastal Plain -Sedimentology, Stratigraphyand Hydrogeology, Special Volume, Elsevier.Harris, W.B., Zullo, V.A. and Baum, G.R., 1979, Tectonic effects onCretaceous, Paleogene, and early Neogene sedimentation,North Carolina; in G.R. Baum, W.B. Harris and V.A. Zullo,eds., Structural and Stratigraphic Framework for the CoastalPlain of North Carolina: Carolina Geological Society and theAtlantic Coastal Plain Association, Field Trip Guidebook,Wrightsville Beach., North Carolina, p. 17-29.Harris, W.B., Zullo, V.A., and Laws, R.A., 1993, Sequence stratigraphyof the onshore Palaeogene, southeastern Atlantic CoastalPlain, USA; in H.W. Posamentier, C.P. Summerhayes, B.U.Haq and G.P. Allen, eds., Sequence Stratigraphy and Facies7

W. Burleigh HarrisAssociations: Special Publication of the International Associationof Sedimentologists, v. 18, p. 537-561.Laws, R.A., 1992, Correlation of Cenozoic continental marinedeposits in North and South Carolina to standard calcareousnannofossil and diatom zonations; in V.A. Zullo, W.B. Harrisand V. Price, eds, Savannah River Region: Transition Betweenthe Gulf and Atlantic Coastal Plains. Proceedings of the SecondBald Head Island Conference on Coastal Plains Geology, Universityof North Carolina at Wilmington, Wilmington, p. 110-116.Laws, R.A. and Worsley, T.R. 1986, Onshore/offshore Oligocenecalcareous nannofossils from southeastern North Carolina:Geological Society of America, Abstracts with Programs, v. 18,p. 251.Mancini, E.A. and Tew, B.H., 1988, Paleogene stratigraphy andbiostratigraphy of southern Alabama: Field Trip Guidebook forthe GCAGS- GCS/SEPM, 38th Annual Convention, NewOrleans, Louisiana, 63 p.McCartan, L., Owens, J.P., Blackwelder, B.W., Szabo, B.J.,Belknap, D.F., Kriausakul, N., Mitterer, R.M., and Wehmiller,J.F., 1982, Comparison of amino acid racemization geochronometrywith lithostratigraphy, biostratigraphy, uranium-seriescoral dating, and magnetostratigraphy in the Atlantic CoastalPlain of the southeastern United States: Quaternary Research, v.18, p. 337-359.McCartan, L., Lemon, E.M., and Weems, R.E, 1984, Geologic mapof the area between Charleston and Orangeburg, South Carolina:U.S. Geological Survey Miscellaneous InvestigationSeries Map 1-1472.Mixon, R.B. and Pilkey, O.H., 1976, Reconnaissance geology ofthe submerged and emerged Coastal Plain Province, CapeLookout area, North Carolina: U.S. Professional Paper 859, 45p.Owens, J.P., 1989, Geologic map of the Cape Fear region, Florence10 x 20 Quadrangle and northern half of the Georgetown 10 x20 Quadrangle, North Carolina and South Carolina: U.S. GeologicalSurvey Miscellaneous Investigation Series Map 1-1948-A.Parker, W. and Laws, R.A., 1991, Calcareous nannoplankton biostratigraphyof the exposed and subsurface Oligocene and lowerMiocene strata in southeastern North Carolina: GeologicalSociety of America, Abstracts with Program, v. 23, p.113.Riggs, S.R. and Mallette, P.M., 1990, Patterns of phosphate depositionand lithofacies relationships within the Miocene PungoRiver Formation, North Carolina continental margin; in W.Burnett and S.R. Riggs, eds., Phosphates of the world, v. 3:Cambridge University Press, Cambridge, p. 424- 445.Rossbach, T.J. and Carter, J.G., 1991, Molluscan biostratigraphy ofthe Lower River Bend Formation at the Martin Marietta Quarry,New Bern, North Carolina: Journal of Paleontology, v. 65, p.80- 118.Soller, D.R. and Mills, H.H., 1991, Surficial geology and geomorphology;in J.W. Horton and V.A. Zullo, eds., The Geology ofthe Carolinas: University of Tennessee Press, Knoxville, Tennessee,p. 290- 308.Snyder, S.W. and Riggs, S.R., 1993, Geological overview of LeeCreek Mine and vicinity, North Carolina Coastal Plain: TheCompass, Earth Science Journal of Sigma Gamma Epsilon, v.70, p. 13-35.Snyder, S.W., Snyder, S.W., Riggs, S.R., and Hine, A.C., 1991,Sequence stratigraphy of Miocene deposits, North Carolinacontinental margin; in J.W. Horton and V.A. Zullo, eds., TheGeology of the Carolinas: University of Tennessee Press,Knoxville, Tennessee, p. 263-273.Ward, L.W., Bailey, R.H., and Carter, J.G., 1991, Pliocene and earlyPleistocene stratigraphy, depositional history, and molluscanpaleobiogeography of the Coastal Plain; in J.W. Horton andV.A. Zullo, eds., The Geology of the Carolinas: University ofTennessee Press, Knoxville, Tennessee, p. 274-289.Worsley, T.R. and Turco, K, 1979, Calcareous nannofossils fromthe lower Tertiary of North Carolina; in G.R. Baum, W.B. Harris,and V.A. Zullo, eds" Structural and stratigraphic frameworkfor the Coastal Plain of North Carolina: Carolina GeologicalSociety, 1979 Field Trip Guidebook, p.65-72.Zarra, L. 1989, Sequence stratigraphy and foraminiferal biostratigraphyfor selected wells in the Albemarle Embayment, NorthCarolina: Open- file Report, North Carolina Geological Survey,Department of Environment, Health and Natural Resources,No.89-5, 48 p.Zullo, V.A. and Harris, W.B., 1979, Plio-Pleistocene crustal warpingin the outer Coastal Plain of North Carolina; in G.R. Baum,W.B. Harris and V.A. Zullo, eds., Structural and StratigraphicFramework for the Coastal Plain of North Carolina: CarolinaGeological Society and the Atlantic Coastal Plain Association,Field Trip Guidebook, Wrightsville Beach, North Carolina, p.31-40.Zullo, V.A. and Harris, W.B., 1987, Sequence stratigraphy, biostratigraphyand correlation of Eocene through lower Miocenestrata in North Carolina; in C.A. Ross and D. Haman, eds.,Timing and Depositional History of Eustatic Sequences: Constraintson Seismic Stratigraphy: Cushman Foundation for ForaminiferalResearch, Special Publication 24, p. 197-214.Zullo, V.A. and Harris, W.B., 1992, Sequence stratigraphy ofmarine Pliocene and lower Pleistocene deposits in southwesternFlorida; preliminary assessment; in T. M. Scott and W. D. Allmon,eds., The Plio-Pleistocene stratigraphy and paleontologyof southern Florida: Florida Geological Survey, Special Publication36, p. 27-40.8

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 9 - 18THE COQUINAS OF THE NEUSE FORMATION, NEW HANOVER COUNTY, NORTH CAROLINAJames A. DockalDepartment of Earth SciencesUniversity of North Carolina at WilmingtonWilmington, NC 28403-3297ABSTRACTThe coquinas of the Neuse Formation in southern NewHanover County, North Carolina, represent only a small portionof a depositional suite, which formed in the high-energyenvironment of a Late Pleistocene shoreface at a time correspondingto oxygen isotope stage 3 or 75 to 55 ka BP. Thefauna associated with the coquina indicates climatic conditionsthat are i indistinguishable from the present climate.The coquinas are the product of post-depositional diagenesisof carbonate shell bearing shoreface sands where dissolution,cementation, and calcification of aragonite occurred at ornear the paleo-water table. A later dissolution episode of thecarbonate fraction of the coquina and associated strata byoxygenated meteoric waters resulted in the formation of anunlithified, generally reddish, non-fossiliferous sand whichgenerally blankets the area. IINTRODUCTIONFigure 1. Map of the study area showing the occurrences ofcoquina both on shore and off shore in southern New HanoverCounty, North Carolina. A-A’, B-B’, and C-C’ indicate thelocations of profiles illustrated on Figure 7.The coquina found on the beach in the area of FortFisher in New Hanover County, North Carolina, representsone of the very few naturally occurring rock out croppings inthe Coastal Plain Province of the Carolinas. The coquina isnot laterally extensive nor is it of significant thickness, but itoccurs as sporadic isolated patches in a north to south arcuateband over an area roughly 15 km long by 2 km wide (Figure1}. This paper presents a synopsis of the stratigraphicnomenclature applied to the coquina and then presents adetailed petrologic description and interpretation of the conditionsof deposition and diagenesis of the coquina.The published record of coquinas in southeastern NorthCarolina and especially New Hanover County is sparsealthough the area has been visited frequently by geoscientistsfor over 200 years. The first published record is in Stephenson's(1912} description of the Pamlico Formation under"Detailed Sections" where he notes the presence of coquinarock at "Old Fort Fisher" and at a site “one mile southeast ofCarolina Beach wharf." Stephenson's report, however, onlyprovides a brief list of some of the fauna collected by a Dr.Vaughan and provides a photograph of the outcrop on thebeach at Fort Fisher. U. S. Army engineers, during the courseof a beach erosion study in 1931 made 14 "wash borings" atthe Fort Fisher site (House Document 204, 72 ND Congress,1 st Session). These provide some insight into the lateralvariability of the strata though the records lack detailed lithologicdescriptions. Richards (1936), in describing the faunaof the Pamlico Formation, mentioned the exposures at SnowsCut and Fort Fisher and provided a comprehensive list of thefauna. Wells (1944) provided the first detailed description ofthe Pleistocene strata in the Carolina Beach-Fort Fisher area.He divided the strata into five units: Galveston Sand, PineSand, Castalia Sand, Kure Sand, and Cape Fear Coquina.This is apparently the first in print usage of the term .'CapeFear Coquina." Fallow and Wheeler (1969) in their definitionof the Neuse Formation noted several locations ofcoquina in the Carolina Beach area. They designated thecoquina as representing the .'Coquina facies" of the Neuse9

James A. DockalFormation and they designated the section on the north sideof Snows Cut as a reference section for the proposed NeuseFormation. Fallow (1973) later considered the coquina to bethe "High-energy Facies" of the Neuse Formation. Moorefield(1978 was the first to recognize and describe submergedcoquina outcrops in the area. He noted that thesubmerged outcrops, though identical to those on the landsurface, were encrusted with algae, barnacles, colonies of thebivalve Mytilus, serpulid worms, bryozoans and the coralAstrangia. Moorefield reported that these submerged outcropswere undergoing biological erosion especially by therock- boring bivalve Lithophaga. The U. S. Army Corps ofEngineers (1982), during the course of a design study forbeach stabilization, made a number of split spoon borings atthe Fort Fisher Historic site. Some of these penetrated to theunderlying Castle Hayne Limestone (Eocene). The recordsof these borings in combination with the 1931 records providea good view of the lateral variance in lithology over avery limited geographic area. Prosser (1993) attempted toascertain the age of the coquina by using the Uranium seriesmethod on Mercenaria shells collected at Snows Cut.Dockal (1992, 1995b) applied the radiocarbon method toshells of Donax variabilis, Nassarius obsoleta, and Crassostreavirginica, which were also collected at Snows Cut. Wehmillerand others (1988) and Wehmiller and others (1995)applied the amino acid racemization to specimens of Mercenariafrom the coquina.NOMENCLATURE REVIEWThe fossiliferous sands of Pleistocene age in NewHanover County and adjacent offshore areas are informallyreferred to today as the "Cape Fear Coquina," a term firstused by Wells (1944) for the coquinas between Snows Cutand Fort Fisher. This term can not be used as a formal lithostratigraphicname because the name Cape Fear is already inuse for Cretaceous strata located in the coastal plain of theCarolinas; the Cape Fear Formation.The earliest workers within the southeastern Atlanticseaboard region applied the term Columbia Formation andlater Columbia Group to all the Quaternary strata. Bothterms have not been applied in the region for decades andwere never used in reference to the strata encompassed bythis study outside of Stephenson's (1912) usage of PamlicoFormation which was considered at that time to be a subdivisionof the Columbia Group. Stephenson (1912) and laterRichards (1936) definitely referred to the strata considered inthis report as belonging to the Pamlico Formation; howeverlater workers did not make use of the Pamlico name in alithostratigraphic sense (Figure 2). Du Bar and Solliday(1963) argued not to use the term Pamlico Formation partlybecause the type area was a terrace plane, a geomorphic featureand therefore the unit did not represent a true lithostratigraphicunit. Du Bar and Solliday (1963) proposed theFigure 2. Lithostratigraphic nomenclature that has beenapplied in the literature to the coquina and associated strata insouthern New Hanover County.Fanner Beach Formation to replace the concept of the Pamlicoas a lithostratigraphic unit. The name Fanner Beach hasnever been applied directly to the coquinas of the Cape Fear.Fallaw and Wheeler (1969) objected to usage of the FannerBeach Formation because the name included an assemblageof "distinct lithologic units" or units of terrestrial origin andthose of clearly marine character. Fallaw and Wheeler (1969)proposed the name Neuse Formation which was by definitionto encompass just the "marine fossiliferous Pleistocenedeposits in North Carolina." This they divided into fourfacies: "Fine-grained sand facies", Very fine-grained sand10

COQUINAS OF THE NEUSE FORMATIONfacies", Sand-silt-clay facies", and "Coquina facies." Theoutcrop on the north side of Snows Cut was designated as areference section for their Neuse Formation. Du Bar and others(1974) proposed the name Socastee Formation for agroup of related strata near Myrtle Beach, South Carolina.They indicate that the Socastee is found within the Wilmington,North Carolina area but did not specify where. Thedescription of the Socastee is very similar to that of the stratareported here and to that of the Neuse Formation. It isbelieved here that the Socastee Formation is in synonymywith the Neuse Formation, differing only in the state inwhich each are found. The name Neuse Formation thereforehas priority over the name Socastee Formation when beingapplied to North Carolina strata.Owens (1989) applied the name Wando Formation tothe sands at Snows Cut and Waccamaw Formation to thecoquina. Use of the term Wando is valid for the surficialunlithified sands which overlie the coquina; but applicationof the term Waccamaw to the coquina is wrong. The WaccamawFormation occurs at a depth of -10 meters (-35 feet)below mean sea level and below the coquina exposuresthroughout the area as indicated by drilling at Fort Fisher bythe U. S. Army Corps of Engineers (1982). Zarra (1991)referred to the coquina as the "Fort Fisher Coquina" butmade no attempt to formerly define the name. Dockal(1995b) applied the informal name "Cape Fear Coquina" anddivided it into three subdivisions or lithofacies of diageneticorigin; "shell hash lithofacies", sandy limestone lithofacies",and "Kure sand." The later being equivalent to Wells (1944)use of the term "Kure Sand" and the other two being equivalentto his "Cape Fear Coquina." Dockal's incorporation ofthe "Kure sand" into the Cape Fear Coquina was based uponthe interpretation that it represented the insoluble residue leftafter the leaching of the carbonate shells and cements of thecoquina; a view also suggested by Fallaw and Wheeler(1969). It is recommended here that the name Cape FearCoquina be suppressed and that name Neuse Formation beapplied both to the coquinas as originally intended by Fallow11

James A. DockalFigure 3. Stratigraphic section from the North Side of Snows Cut. Unit 1 is below sea level and is inferred from drilling data.Mean low is roughly at the base of Unit 2.12

COQUINAS OF THE NEUSE FORMATIONand Wheeler (1969) and also to the strata in the area that aredepositionally related to the coquina as described below.DESCRIPTION OF THE COQUINAS ANDRELATED BEDSThe best exposure of the coquina and associated strata isto be found on the north bank of Snows Cut west of the US421 bridge (Figure 1) (Site 24 of Carter and others, 1988).The coquina at Snows Cut ranges from a medium to verycoarsely grained fossiliferous sand to an arenaceous fossiliferouslimestone; the siliciclastic fraction ranges from 32% to54% of the mass (Table 1 ). The dominant grains are subangularto rounded monocrystalline quartz and well roundedmolluscan shell fragments. These are weakly to moderatelycemented with blocky calcite. Molds after various aragoniticbioclasts are abundant but aragonitic shells and shell fragmentsare still in abundance as well as shells of calcitizedaragonite. Grain size, though moderately to well sortedthroughout, is not constant (Table 1). There are coarser zoneswhich contain whole molluscan shells, especially Mercenaria,Busycon, Crassostrea, and Rangia. These zones areup to 0.5 meters thick and traceable across the whole outcrop.Shells are generally concave down and imbricate suggestinga southerly transport direction. Trough crossstratification,though weakly marked, also indicates a southerlytransport direction. The coquina at the west end of thenorth bank of Snows Cut has a maximum thickness of 1 .5meters and pinches rapidly eastward and westward. Those onthe south bank are over 2 meters thick but generally present apoorer exposure.The coquina at Snows Cut is overlain by an unlithifiedarenaceous shell hash or marl (Figure 3). Dominant grainsare like that of the coquina; monocrystalline quartz androunded shell fragments with the siliciclastic fraction representing34% to 46% of the mass (Table 1 ). Grain size distributionsof the siliciclastic fraction are comparable to those ofthe coquina (Table 1 ). Scattered throughout are well-preservedmolluscan shells, but none are in growth position,none are articulated, and most but not all exhibit somedegree of abrasion (Dockal 1995b). Like the coquina there isa weak sense of imbrication and cross-stratification. Cementis absent except for a minor amount of pendant cementwithin some of the shelter pores and meniscus calcite cementnear the base of the arenaceous shell hash.The contact with the underlying coquina is a rapid gradationin the degree and type of cementation. Dockal(1995b) interpreted this to have resulted from diagenesisnear the paleo-water table. The shell hash proper with itspendent cements lying within the vadose lone, the coquinawith its blocky calcite cement lying within the phreatic lone,and the boundary area with the meniscus cement representthe capillary fringe just above the water table.Overlying the shell hash and the coquina where the shellhash is absent is a medium to coarse grained, moderately topoorly sorted, near symmetrical to coarse skewed, platykurticto leptokurtic sand (Table 1 }. The sand is nonindurated tomoderately indurated with ferruginous cement. The color ofthe sand ranges from a pale yellow to black. Sedimentarystructures are generally absent but one can observe zoneswhere larger grains, granules and pebbles occur within thestill poorly sorted sand. These when tracked laterally passinto the coarser shell bearing zones of the shell hash orcoquina. On the north side of Snows Cut and about halfwaybetween the main coquina outcrop and the US 421 bridge alens of finer and better sorted sand occurs within this sandlayer (Table 1, samples SCN-40, 51, 55}. This lense hasweakly marked trough cross-stratification and some burrows.The boundary between these two sand types is very indistinctsuggesting that the finer sand my have been a lowerenergy phase with less carbonate bioclasts. At the easternend of Snows Cut several dark brown to black horizons canbe seen within the sand layer. These appear to be only zonesof enrichment of iron oxides and have no relation to a sedimentarystructure or to fossil soil horizons.The contact between the sand and the underlying shellhash or coquina is a very distinct and undulatory surfacewhere the overlying sand appears to literally interfinger in avertical sense with the underlying shell hash or coquina. Thesand forms cylindrical bodies 0.2 to 0.4 meters in diameterwhich project into the underlying material a meter or more.Dockal (1995b} described these as "fingers" and Moorefield(1978} referred to them as "potholes." They represent ageochemical front along which carbonated material dissolved.The sand, which sits above this surface, is the insolubleresidue left over after dissolution of all the carbonatefrom the shell hash or coquina. The sand has the same graintypes and grain size distribution as that of the insoluble residuesfrom the shell hash and coquina (Table 1 }.At the top of this sand layer is a well-marked nearly horizontallying paleosol. Overlying this is finer grained sandthat is moderately to well sorted. This sand is directly relatedto some of the Holocene dunes in the immediate area such asSugar Loaf within Carolina Beach State Park just to thesouth of Snows Cut. Overlying this are spoils from the constructionof the canal.What underlies the sand and where present, the coquinaat Snows Cut is not as well understood. Recent drilling bythe U.. S. Army Corps of Engineers just east of the bridgerevealed 610 7 meters of gray green sands with abraded shelldebris and bands of shells and greenish gray silty sand sittingbelow the ferruginous sand (Ben Lackey, personal communication1996). This material appears to be little different outsideof color from the main sand noted above and theunlithified shell hash.The coquina and associated beds at Fort Fisher (Figure1) are very similar to those of Snows Cut. The cross stratificationof the coquina is easier to see but this is more a reflec-13

James A. DockalFigure 4. Wash boring logs of Line “A” from the beach face atFort Fisher Historic Site. (Modified from House DocumentNo. 204, 72 nd. Congress, 1 st. Session, Plate VII).tion of outcrop type and not a true lithologic difference withthat of Snows Cut. The cross- stratification at Fort Fisher hasbeen described in detail by Fallow and Wheeler (1969), Fallow(1973), and Mansfield (1978). Mansfield (1978) foundthree sets of directions indicating paleocurrent flows ofS65°W, S23°E and S22°W. As at Snows Cut the coquina isnot laterally extensive. In comparison between the 1931 borings,the 1982 borings, and the present outcrop it appearsthat the main body of coquina was to the east of the presentbeach and has since 1931 largely eroded away. The 1931study indicated that the coquina was as much as "9 feetthick" in places and extended continuously from boring toboring north to south (Figure 4). U.S. Army Corps of Engineers(1982) reports that "The coquina is irregular in thicknessand elevation" and that "it is also discontinuous " Theshell hash observed overlying the coquina at Snows Cut hasnot been observed in recent times at Fort Fisher however the1931 study indicates a similar lithology being present. Theferrugenous sand that dominated the Snows Cut outcrop ispresent as well as the paleosol at the top of the sand. Well's(1944) description of this area notes several sand and associatedpaleosols. However these appear now to be no differentthat the color banding noted at Snows Cut in the main sandlayer. Underlying the coquina is a plastic greenish- gray siltwhich contains sand filled burrows. The 1982 Corps of Engineersborings indicate 10 meters (32 feet) of gray to greensand similar to that found at Snows Cut underlie the coquinaand associated ferruginous sand. The change from the ferrugenousdark brown to buff sands to the gray green sandtype "is irregular with elevation from boring to boring varyingas much as 5 feet and ranging from zero to -10 m.s.l."U.S. Army Corps of Engineers (1982).The coquina occurs at other scattered localities throughoutthe area from just north of Snows Cut to Fort Fisher andadjacent off shore waters (Figure 1 ). Lithology of thecoquina is always the same and in the on shore areas it isalways associated with the ferruginous sand. The coquina,the shell hash, and the surrounding sands, which lie belowthe paleosol, are all considered here to be of the same originaldepositional layer. There present difference in lithologybeing due entirely to post-depositional diagenesis. The initialsediment was probably much like the shell hash seen atSnows Cut or the finer sand noted also at Snows Cut andfrom the Corps of Engineers borings. The coquina is simplycemented shell hash where the cementation formed at or nearthe paleo-watertable. The ferrugenous sand is the insolubleresidue left from the later dissolution of all carbonate materialfrom the shell hash, coquina, or finer sand. The reddishcoloration represents the zone of oxidation of the sedimentswhich is probably modern or Holocene. This passes downwardto the greenish gray colored sediments found in theborings which are situated in a reducing geochemical environment.Lithoclasts found in the coquina, shell hash, and sandsare the same. These range in size from granules to cobblesand vary in lithology ranging from metamorphic rock fragmentsderived from Piedmont source areas to sedimentaryrock fragments of Coastal Plain sources. Most are wellrounded and half have a discoidal shape which would becharacteristic of a pebble that has been within the surf environmentfor some time. Of the sedimentary rock fragmentsmost probably were derived from sandstone in the Pee DeeFormation (Cretaceous) which would have been exposed afew tens of kilometers northwest of the area near Wilmington.One lithoclast contained the oyster Conradostrealawrencei which would have been derived from the WaccamawFormation (Pliocene/ Pleistocene). The Waccamawoccurs in the subsurface in the area about 11 meters (35 feet)below the top of the coquina and probably was exposedwithin a few kilometers northwest of the area. Of particularinterests are lithoclasts of coquina found within the coquina.These range up to 15 cm in diameter, are somewhat rounded,spherical, and identical in all aspects of lithology to the mainbody of coquina. This indicates that the area has or had morethan one body of coquina and further points out that thecoquina and associated beds contain material reworked fromolder beds.Mollusks which are commonly found in the coquina andshell hash include: Anadara brasiliana, Crassostrea virginica,Dionocardium robustum, Donax variabilis, Mercenariamercenaria, Rangia cuneata, Tagelus plebius, Busycon con-14

COQUINAS OF THE NEUSE FORMATIONFigure 5. Modern latitude ranges for the molluscan fauna fromthe Neuse Formation of Snows Cut, New Hanover County,North Carolina (34°× 3’14” N; 77°× 54’23” W).trarium, and Nassarius obsoletus. Other identified mollusksare listed in Figure 5 and Richards (1936), Fallaw (1973),and (Dockal 1995b). The foraminifera fauna is dominated byOuinqueloculina and Hanzawaia (Fallaw, 1973) The twocorals present are, Septastraea crassa which is quite commonthough always greatly abraded and Siderastrea radianswhich is rare but always well preserved, lacking abrasion,and encrusting other bioclasts. The barnacle Balanus improvisushas been described from the strata (Zullo and Miller,1986). Flat echinoid fragments are common and an occasionalwhole specimens of the sand dollar, Millita sp., arefound. Small fish teeth and crab claws are readily found inthe finer fractions and shell fillings. Mammalian bones andteeth are infrequently encountered. These include mastodon(Richards, 1936), bison (Wilmington Star News, February4,1995), camel and deer.AGE OF THE COQUINAS AND RELATED STRATAThe age of the coquina has generally been considered tobe Late Pleistocene (Fallaw and Wheeler, 1969). Thecoquina contains Anadara brasiliana (Lamarck), a molluskswhich is characteristic of Blackwelder's (1981) upper Pleistoceneand Holocene Yongesian Substage of the LongianMolluscan Stage. The present near sea level topographicposition of these deposits suggest deposition in associationwith a sea level high stand above the altitude of the presentsea level but below that of the high stand associated with thenearby 7-meter (25 foot) terrace. The Yongesian Substagedesignation for the coquina, its geomorphic position east orseaward of what is possibly the Suffolk Terrace, and the elevationof the strata relative to sea level suggests assignmentto Stage 5 of the oxygen isotope base sea level curve ofChappel and Shackleton (1986). Wehmiller and others(1988), applying amino acid racemization, found a mean D/L Leu value of 0.52 from two specimens of Mercenaria sp.which were collected at Snows Cut. This value correspondsto amino zone Illd of Wehmiller and others (1988) whichapparently has an age in excess of 220 ka.. Recent work byWehmiller and others (1995) notes the additional amino acidanalysis of 16 specimens of Mercenaria sp. from Snows Cut.The AI I values of these "cluster into two apparent aminozoneswith mean values of approximately 0.46+/-0.025(n=4) and 0.34+/-0.025 (n= 12)" (Wehmiller and others,1995, p.331). The lower ratio corresponds to oxygen isotopestage 5 the higher ration to stages 7 to 9 (Wehmiller and others,1995, figure 2). Dockal (1995b) argued that the abundanceof fossils reworked from older units and the presenceof purple colored Mercenaria, which elsewhere in the regionhave lower A/l values than 0.34, would imply that even lowerA/l values should be obtainable from the Snows Cut strataand therefore they should represent an age younger thanstage 5. Uranium series dating conducted by Prosser (1993)on Mercenaria sp. shells from the shell hash at Snows Cutindicate an age of 62 ka BP. Radiocarbon evaluation of avariety of shells from the same site and stratum at Snows Cutfound an apparent age of 24 to 29 ka BP (Dockal 1992,1995b). However, as Dockal (1995a) pointed out radiocarbonassays giving this range of apparent age may have beenaffected by an extreme enhancement of the cosmic ray fluxand thus may represent an age closer to 60 ka BP. Withoutadditional and improved geochronologic work it is probablysafest to assume that the coquina and related strata at SnowsCut and Fort Fisher were deposited before the most recentglaciation, isotope stage 2, yet sometime after isotope stage5. This would best correspond to a slight warming period andsea level high stand belonging to isotope stage 3 or 75 to 55ka BP.ENVIRONMENT OF DEPOSITIONThe molluscan fauna suggest deposition under climaticconditions similar to that found today along the southeasternAtlantic seaboard between 34 and 36 degrees north latitude(Figure 5). Fallaw (1973) considered the molluscan fauna ofthe Neuse Formation as a whole to indicate climate conditionssimilar to those of today but with slightly higher watertemperatures than present. Part of the evidence of this wasthe presence of Cardita floridana, Pyramidella crenulata,and Cantharus cancellarius. The present northern limit ofthe range of the former is Florida and the later two is South15

James A. DockalFigure 6. Diagrammatic stratigraphic profiles. I. corresponds to the Holocene or modern (Isotopes Stage 1) beach and slat marshsediments. III. (shaded) corresponds to the coquina and associated strata (Isotope Stage 3). V? corresponds to earlier strata (IsotopeStage 5 and older). All three profiles trend normal the present beach face. A-A’ is located at the top of the area covered by Figure 1.B-B’ passes through the center of Snows Cut. C-C’ passes directly through Fort Fisher. Surface profile taken from 7.5 minute topographicmaps; subsurface control is very minimal. Please note vertical exaggeration is approximately 100 times.Carolina. All three of these are, however, absent from thecoquinas of New Hanover County and were only reported byFallow (1973) from the Neuse estuary exposures of theNeuse Formation. If these coquinas are truly coeval with theNeuse estuary exposures then the lack of these taxa could beexplained as a result of differing environmental niches. Onthe other hand if they are not coeval then that leaves open thepossibility that water temperature during deposition of thecoquina was not necessarily warmer than present.The fauna, based upon the ecological ranges of theirmodern counterparts, represents a mixture of taxa from severalenvironments: salt marsh, beach surf zone or intertidal,and shallow shelf (water depth less that 10 meters). The presenceof a coral which is effectively in growth position andarticulated sand dollars indicates shallow subtidal waterdepth and normal marine salinity (see Heckel, 1972). Furthermorethe corals and to a lessor extent the barnacles implyclear water. The dominant foram, Quinqueloculina, is arobust thick wall form that would be expected to thrive in ahigh-energy environment (Fallaw 1973). The presence of theclam Rangia and the numerous lithoclasts of both Piedmontand nearby sedimentary rock units suggests close proximityto a fluvial system. The terrestrial mammalian remains mayalso have been transported via a river or they could representfauna living close to the sediment depositional site. Fallaw(1973) considered the environment of deposition to be one ofhigh energy possibly a shoal or tidal inlet. Moorefield (1978)suggested that "the coquina may have been deposited in amigrating Cape Fear River mouth, which is essentially a"mega-inlet'." Dockal (1995b) envisioned the coquina andshell hash exposed on the north side of Snows Cut to haveresulted from a single storm event where storm debris weredeposited above the level of high tide. None of these studieshave taken into consideration the bigger picture. The boreholesin the area indicate at least 1 p meters of sedimentassociated with this package. Furthermore they suggest multipledepositional events of both a high energy and lowenergy nature. These strata extend north to south for about15 kilometers forming an arcuate shaped package (Figure 1).In cross-sectional profile (Figure 6) they closely resemble16

COQUINAS OF THE NEUSE FORMATIONthe modern and ancient barrier systems in the region differingonly in their rather limited lateral extent and lack of associatedback barrier salt marsh sediments. The interpretationof environment of deposition favored here is one of shorefacesediments deposited from the level of the subaeriallyexposed beach to below the level of low tide, where depositionoccurred within a largely sediment starved basin. As sealevel rose the shoreface environment shifted laterally landwardby storm wave action eroding sediment from the foot ofthe shoreface and redepositing it at the top, on the subaerialbeach. The deposited material was a mixture of old reworkedsediment from previous beach face deposits and what fluvialsediment that had accumulated after the last high stand Theresultant fossil assemblage consisted of varying amounts ofin situ forms such as Donax variabilis, proximal indigenousforms such as Crassostrea virginica and Nassarius obsoletus,distal indigenous forms like Aequipectin gibbus and Siderastrearadians, exotic forms like the terrestrial mammalianbones, and remanie forms reworked from older strata as evidencedby the amino acid racemization results of Wehmillerand others (1995). The modern equivalent of this is the beachdeposits found just south of Fort Fisher. There the sedimenton the beach contains an abundance of modern and fossilshells, blocks of coquina, and debris that has recently arrivedfrom the Cape Fear River.CONCLUSIONSThe term "Cape Fear Coquina" should be suppressedand in its place the name Neuse Formation should be usedboth for the coquina and the associated arenaceous shell hashand carbonate free sands. The coquinas represent a post depositionaldiagenetic product of a shell bearing sand depositedin a shoreface environment. Initial diagenesis took place ator near an ancient water table where dissolution of aragoniticbioclasts gave rise to calcite spar cements and the formationof the coquina. The ferrugenous sands which were in the pastwere referred to as the "Kure Sand" also represent a productof the diagenesis of both the coquinas and the associatedunlithified sands which were the precursors to the coquina.Here. diagenesis took place during subaerial exposure whereoxygenated meteoric waters both dissolved calcium carbonateand oxidized what iron bearing heavy minerals werepresent, resulting in a ferrugenous stained insoluble residue.The exposed coquinas and ferrugenous residue sands representonly the present surficial expression of a much thickersediment package of limited lateral extent. These weredeposited during a minor sea level high stand whichoccurred after isotope stage 5 but before stage 2, a period oftime dating from approximately 75 ka to 55 ka beforepresent or isotope stage 3.ACKNOWLEDGMENTSI am grateful to William J. Cleary and William B. Harrisfor their reviews of the manuscript, to Victor Zullo for hisintroduction to the coquina and its fauna, and to JodyDuMond, who as a high school student assisted in the collectionand identification of the fossils from Snows Cut.REFERENCESBlackwelder, B.W., 1981 Late Cenozoic stages and Molluscansones of the U.S. Middle Atlantic Coastal Plain. Journal ofPaleontology, v. 55, pt. II of II, supplement to no.5; PaleontologicalSociety, Memoir 12, 34 p.Carter, J. G., Gallagher, P. E., Valone, R. E., and Rossbach, T. J.,1988, Fossil Collecting in North Carolina: North CarolinaDepartment of Environment, Health, and Natural Resources,Division of Land Resources, Geological Survey Section Bulletin89.Chappel, J. and Shackleton, N.J., 1986 Oxygen isotopes and sealevel. Nature v. 324, p. 137-140.Dockal, J. A., 1992 Radiocarbon dating of late Pleistocene marinedeposits, New Hanover County, North Carolina. GeologicalSociety of America Abstracts With Programs, v. 24, No.2, p.12.Dockal, J. A., 1995a Evaluation of an apparent Late Pleistocene(25-40 ka BP) sea level high stand: An artifact of a greatlyenhanced cosmic ray flux of -60 ka BP. Journal of CoastalResearch, v. 11, No.3, p. 623-636.Dockal, J. A. 1995b Documentation and evaluation of radiocarbondates from the "Cape Fear Coquina" (Late Pleistocene) ofSnows Cut, New Hanover County, North Carolina. SoutheasternGeology, v. 35, No.4, p. 169-186.Du Bar, J.R., Johnson, H.S., Jr., Thorn, B, and Hatchell, W.O., 1974Neogene stratigraphy and morphology, south flank of the CapeFear Arch, North and South Carolina. In: Oaks, R.Q. and DuBar, J.R. (editors) Post-Miocene stratigraphy centra and southernAtlantic Coastal Plain. Utah State University Press, Logan,Utah, p. 139-173.Du Bar, J.R. and Solliday, J.R. 1963 Stratigraphy of the Neogenedeposits, lower Neuse estuary, North Carolina. SoutheasternGeology, v. 4, p. 213-233.Fallaw, W. 1973 Depositional environments of marine Pleistocenedeposits in southeastern North Carolina. Geological Society ofAmerica Bulletin, v. 84, no.1, p. 257-268Fallaw, W. and Wheeler, W.H., 1969, Marine fossiliferous Pleistocenedeposits in southeastern North Carolina. SoutheasternGeology, v. 10, no. 1, p. 35-54.Folk, R. L., 1980, Petrology of Sedimentary Rocks: Hemphill PublishingCo., Austin, Texas, 182 p.Heckel, P.H. 1972. Recognition of Ancient Shallow Marine Environments.In: Rigby & Hamblin (editors) Recognition ofAncient Sedimentary Environments. SEPM Special PublicationNo.16, p. 226-286.Mixon, R.B., 1986 Depositional environments and paleogeographyof the intergalcial Flanner Beach Formation, Cape Lookoutarea, North Carolina. Geological Society of America CentennialField Guide-Southeastern Section, p. 315-320.Mixon, R.B. and Pilkey, O.H., 1976 Reconnaissance geology of the17

James A. Dockalsubmerged and emerged Coastal Plain Province, Cape Lookoutarea, North Carolina. U.S. Geological Survey ProfessionalPaper 859, 45 p.Moorefield, T P., 1978, Geologic processes and history of the FortFisher coastal area, North Carolina, Unpublished Masters thesis,East Carolina University, Greenville NC, 100 p.Owens, J. Po, 1989, Geologic map of the Cape Fear region, Florence10 X 20 Quadrangle and northern half of the Georgetown10 X 20 Quadrangle, North Carolina and South Carolina, U.S.Geological Survey, Miscellaneous Investigations Map 1-1948-A.Prosser, J. F., 1993, Apparent uranium-series dates for mollusksfrom Snow's Cut, North Carolina: Implications for Late Pleistocenechronology, sea- level, and tectonics along the CoastalPlain of Southeastern North Carolina, Unpublished masters thesis,University of North Carolina, Chapel Hill, NC. 44 p.Richards, H. G., 1936, Fauna of the Pleistocene Pamlico Formationof the Southeastern Atlantic Coastal Plain. Bulletin GeologicalSociety of America, v. 47, p. 1611-1656.Stephenson, L. W., 1912, The Quaternary formations, in Clark,W.B., Miller, B.L., Stephenson, L.W., Johnson, B.L., andParker, H.N., The Coastal Plain of North Carolina. North CarolinaGeological and Economic Survey, v. 3, p. 266-290.U.S. Army Corps of Engineers, 1982, Fort Fisher North CarolinaGeneral Design Memorandum Phase II Design Memorandum 2Project Design. U.S. Army Corps of Engineers, WilmingtonDistrict.Wehmiller, J.F., Belknap, D.F., Boutin, B.S., Mirecki, J. E. Rahaim,S.D., and York, L.L., 1988 A review of the aminostratigraphyof Quaternary mollusks from the United States Atlantic CoastalPlain sites. Geological society of America Special Paper 227,p.69-110.Wehmiller, J.F., York, L.L., and Bart, M.L., 1995 Amino acid racemizationgeochronology of reworked Quaternary mollusks onU.S. Atlantic coast beaches: implications for chronostratigraphic,taphonomy, and coastal sediment transport. MarineGeology, v. 124, p. 303-337.Wells, B. W., 1944, Origin and development of the lower Cape FearPeninsula: Elisha Mitchell Science Society Journal, v. 60, no.2,129-134.Zarra, L., 1991, Subsurface stratigraphic framework for Cenozoicstrata in Brunswick and New Hanover Counties, North Carolina,North Carolina Geological Survey Information Circular27.Zullo, v. A. and Miller, W, Ill, 1986, Barnacles (Cirripedia: Balanidae)from the lower Pleistocene James City Formation,North Carolina coastal plain, with the description of anew speciesof Balanus Da Costa: Proceedings Biological Society ofWashington, v. 99(4), 717-730.18

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 19 - 27SHOREFACE PROCESSES IN ONSLOW BAYE. Robert ThielerDuke University, Dept. of GeologyProgram for the Study of Developed ShorelinesDurham, NC 27708-0228THE SHOREFACE ENVIRONMENTThe shoreface is the interface between the continentalshelf and the subaerial coastal plain. The shoreface canbehave as a source, barrier, filter, or conduit for the bi-directionalexchange of materials between the land and the sea.The shoreface of barrier islands is the generally concaveupward surface extending from the surf zone to the pointwhere the slope becomes the same as the very gentle slope ofthe inner and central continental shelf. By this definition, thebase of the shoreface off southeastern North Carolina islocated at 10-12 m water depth (Figure 1).Figure 1. The shoreface is defined as the region between thesurf zone and the inner continental shelf. Off southeasternNorth Carolina, the base of the shoreface is located between 10-12 m water depth. (Modified after Wright et al., 1991).Oceanographic and geologic processes in this environmentdetermine how a shoreline will respond to storms, tosea-level rise and to human-induced changes in sand supplyover time scales from hours to years to millennia. Understandingshoreface processes is also critical to understandingthe behavior of replenished beaches, which provide manybeachfront communities with storm protection, recreationareas, and an important tourism resource. Sediment transportacross the shoreface is a key factor affecting 1) short- andlong-term fluctuations of beach and surf zone sand storage(Wright et al., 1985) ; 2) the morphology and stratigraphy ofthe shoreface (Niedoroda et al., 1985); and 3) the nature ofthe inner shelf sand sheet (Swift, 1976). On retreating barrierisland coasts, the shoreface is also a major source of newsediment to the coastal system, via the erosion and release ofpreviously deposited lagoonal and fluvial sediments. Thisprocess, termed "shoreface bypassing" by Swift (1976), is allthe more important on the southeastern U.S. Atlantic coastdue to the absence of a modern fluvial contribution. Curray(1969) suggested that the present is a unique moment in geologictime with regard to shoreface evolution. Specifically,the relative stillstand in sea-level along most of the U.S. EastCoast since about 4500 BP has allowed shoreface environmentsto mature and steepen as they seek an equilibriumform. If true, this process would minimize the amount ofsediment available to beaches from the continental shelf, andperhaps increase the rate of shoreface retreat.The shoreface is one of the most complex and leastunderstood coastal environments (Wright, 1987; Nummedal,1991). Geologists, oceanographers and engineers are onlyjust beginning to understand that nearly all shoreface environmentsare different, where processes and controls vary inimportance both spatially and temporally (Niedoroda et al.,1985; Kraft et al., 1987; Wright et al., 1991). The shorefaceis also the interface that couples the beach to the shelf. Theoryand empirical observations have done much to identifyshoreface sediment transport rates under various conditions.Presently, however, we can neither identify nor predict thenet transport of material on the shoreface (Wright, 1987;Pilkey, 1993; Nittrouer and Wright, 1994). An appliedunderstanding of shoreface processes is also needed todesign coastal engineering projects, as well as to evaluateand improve models used in coastal engineering to predictthe behavior of beaches. On a millennial time scale, sedimentationon coastal plain shelves during a time of risingsea-level such as the Holocene is driven by the bypassing ofsediment onto the shelf via the shoreface; fluvial sedimentsare trapped in the estuarine system. Swift (1976) describedthis process as "shoreface bypassing." This mechanism providesthe primary source of new sediment to the shelf as theravinement surface bevels previously deposited coastal plainmaterial. Thus, shoreface bypassing regulates sediment sup-19

E. Robert Thielerply, and in effect the rate and character of shelf sedimentation.The shoreface maturation process postulated by Curray(1969) seems to have peaked in Onslow Bay. The regressivebarrier islands (e.g., Shackleford Banks, Bogue Banks andBear Islands) are no longer prograding seaward; they appearto have accumulated all of the available inner shelf sand.This process of "inner shelf sweeping up" appears to havebeen very efficient; much of the inner shelf shows little or noevidence of modem sedimentation (Hine and Snyder, 1985).The transgressive islands (e.g., Topsail Island, WrightsvilleBeach and Masonboro Island) exhibit much the same shorefacecharacteristics, although they have substantially lesssediment volume. Nearly all of the sediment in the TopsailIsland barrier system, for example, is contained within thebody of the island landward of the beach; the shoreface sedimentvolume is very small.Over shorter time frames (e.g., individual storm eventsto several weeks), a variety of processes operate on theshoreface and inner shelf (Figure 2). These processes createhigh bottom stresses that mobilize sediment. As described byGrant and Madsen (1979), the bed agitation required for sedimentresuspension is furnished primarily by gravity waves,but sediment exchange is accomplished by quasi- steadystatemean flows. It is generally recognized that along-shelfflows predominate over across-shelf flows, but that acrossshelftransport gradients are relatively high (Nummedal,1991; Nittrouer and Wright, 1994). Several types of shorefaceand inner shelf currents have been recognized, including:1) storm- driven pressure gradient currents (e.g. windinducedupwelling and downwelling currents); 2) tidal currents;3) storm surge ebb currents; and 4) turbidity currents.Ekman (1905) predicted the presence of inner shelf currentsand Sverdrup, Johnson and Fleming (1942) noted the theoreticalbasis for currents related to pressure fields in theirclassic textbook. Shi and Larsen (1984) and Dean and Perlin(1986) suggested that cross-shore transport on the shorefacecould also be affected by forced long-period (infragravity)waves associated with groupy incident waves.Our contemporary understanding of short-term shorefaceand inner shelf processes is derived from both sedimenttransport modeling and field measurements. Early work onbottom boundary layer models was done by Jonsson {1966).Bijker {1967), and Sternberg {1972). Today, a number ofdifferent models {see review by Dyer and Soulsby. 1988) areused to examine boundary layer structure and sedimenttransport. In the past few years the Grant and Madsen {1979)and Glenn and Grant {1987) models for combined wave andcurrent flow have come into wide use {Cacchione et al.,1994; Nittrouer and Wright. 1994). These models coupledwith field measurements of the bottom boundary layer, haveidentified a number of important factors that influence shorefacesedimentary processes. The rates and directions of sedimenttransport on the shoreface and inner shelf are governedFigure 2. Schematic diagram of the major interacting components of the shallow-water bottom boundary layer. (Modified afterWright et al., 1989).20

SHOREFACE PROCESSES IN ONSLOW BAYprimarily by wave-current interactions {Madsen and Grant,1976); the micromorphology (ripple geometry, biologicroughness. etc.) and sedimentary characteristics of the seabed{Nielsen, 1979; Wright, 1993); and the geologic frameworkof the shoreface {Pilkey et al. 1993; Riggs et al..1995). Figure 2 illustrates the combined effects of these factorsin determining the nature of the bottom boundary layerand the resultant bed stresses and sediment transport.A number of field studies have documented that significantsuspended and bed load transport occurs frequently onthe shoreface and inner shelf. These include Sternberg andLarsen {1975); Gadd et al. {1978); Lavelle et al. {1978);Cacchione and Drake {1982); Vincent et al. {1982); Wibergand Smith {1983); Cacchione et al. {1987; 1994); andWright et al.. {1986; 1991; 1994) among many others. Thesestudies, however, capture only a brief moment in the largescaleevolution of the shoreface. A decade-scale view ofshoreface processes and evolution is currently lacking. Thisis particularly true in engineering studies of coastal processes,which typically require a decade- scale understandingof shoreface evolution.Engineering models used to predict shoreline evolutionand to design replenished beaches usually assume I that theshoreface has an equilibrium shape related I to wave climateand surficial sediment grain size: {Dean. 1977; Zeidler.1982). As applied to the design of replenished beaches, theprofile of equilibrium is considered to be the stable configurationthat a beach will try to achieve under the influence ofincident waves {Dean, 1983). Maintenance of the profile ofequilibrium during shoreline retreat is also central to the conceptof Bruun Rule response to sea-level rise (Bruun, 1962).The equilibrium profile equation was first proposed byBruun (1954) for the Danish North Sea coast, and has theformh = Ayn (1)where h is water depth, y is the distance offshore fromthe shoreline, n is a variable shape parameter and A is a scalingparameter. Bruun (1962) used this equation to develop asimple model for coastal evolution, in which the shorefaceprofile responds to sea-level rise by moving landward andupward such that the profile shape remains constant down toa depth of no wave influence (beyond which little sedimentis supposedly transported). This simple relationship was oneof the first models of shoreface transgression, preceding themore "classic" geologic conceptualizations of Curray (1969)and Swift (1976).The Bruun Rule was, and still is, a good concept. It isnot a good quantitative model. The concept, as originallyconceived by Bruun, provided a strong conceptual basis forfurther thought about the nature of shoreface evolution. Subsequentwork, however, sought to verify its basic principles.For example, Dean (1977) used a least squares approach tofit the data of Hayden et al. (1975) to an equation of the formshown in (1), where n=0.67. Infixing the value of n, Dean(1977) left the sediment scaling parameter, A, as the onlyindependent variable in the equation. Dean (1987) related Ato sediment fall velocity by transforming Moore's (1982)sediment grain size data to the equationA = 0.067 w O.44 (2)where w is the sediment fall velocity in cm S-1. Essentially,this relationship implies that any shoreface profile can bedescribed solely on the basis of the grain size present.The profile of equilibrium concept makes several fundamentalassumptions about the nature of the shoreface and theprocesses acting on it (Dean, 1977; 1991; cf. Pilkey et al.,1993). Pilkey et al. (1993) argued that several basic assumptionsof the shoreface profile of equilibrium concept are notmet in most field settings. The assumptions include: 1) sedimentmovement is driven solely by diffusion due to waveenergygradients across the shoreface; 2) closure depth (aseaward limit of significant net sediment movement) existsand can be quantified; 3) the shoreface is sand-rich, andunderlying geologic framework does not influence the profileshape; and 4) the profile described by the equilibriumprofile equation (Dean, 1977) provides an approximation ofthe real shoreface shape useful for coastal engineeringprojects.The shoreface profile of equilibrium is a fundamentalprinciple behind most analytical and numerical models ofshoreline change used to predict large-scale coastal behavior(e.g., Hanson and Kraus, 1989 [the GENESIS model]) and todesign replenished beaches (e.g., Hansen and Lillycrop,1988; Larson and Kraus, 1989 [the SBEACH model]),including those used on beaches in Onslow Bay. There hasbeen no systematic field verification of the physical basis forthe equilibrium profile equation (Kraft et al., 1987; Wright etal., 1991; Pilkey et al., 1993). The concept, however, hasbeen accepted as valid and useful by many coastal researchers,and is used to predict coastal evolution in a variety ofcoastal settings (e.g., Rosen, 1978).The Bruun Rule effectively states that shoreface slope isthe only control of shoreline retreat and that for a given sealevelrise, beaches with gentle shorefaces will recede fasterthan those with steep shorefaces. In typical applications,retreat rates are based on the slope of the shoreface ratherthan the slope of the migration surface. As a result, on EastCoast shorefaces the Bruun Rule usually predicts a sea-levelrise to shoreline retreat ratio of 1: 200. However, the retreatactually occurs across the surface of the lower coastal plain,the slope of which in southeastern North Carolina, for example,averages about 1: 2000. The Rule is also flawed in itsassumptions concerning areal restriction of sediment movementon shorefaces, and in its lack of consideration for geologiccontrol of shoreface slope. In actual use, theassumption of the depth of no wave motion (closure depth)has decreased to between 4 and 8 m on East Coast shorefaces,in contrast to Bruun's original 18 to 20 m depthassumption. There is no basis in reality for using the Bruun21

E. Robert ThielerRule, as it is currently being used, to predict shoreline retreatrates.A PICTURE OF THE MODERN SHOREFACERecent studies by Duke University, UNC-Wilmington,East Carolina University, NOAA/NURC and the USGS haveexamined the shoreface environments from Fort Fisher toTopsail Island. Over the past six years, extensive geologicand geophysical data has been collected off WrightsvilleBeach, and more recently (1-5 July 1996) off Coke, Lea, andTopsail Islands, which form the basis of the discussionbelow. The dataset includes repeated sidescan-sonar surveysof the outer surf zone, shoreface and inner shelf off WrightsvilleBeach conducted in 1992, 1994, and 1995, as well asover 240 line-km obtained off Topsail Island one week priorto hurricane Bertha. During the 1992 and 1996 sidescan surveys,seismic reflection data was collected (3.5 kHz andUNIBOOM, respectively). We have also obtained a suiteof over 200 short (-2 m) percussion cores, vibracores, surfacesediment samples and diver observations off WrightsvilleBeach, Figure Eight, Coke, Lea and Topsail Islands. Much ofthe area previously covered by sidescan-sonar surveys wasresurveyed after hurricanes Bertha and Fran. Preliminaryresults are summarized below.The transgressive barrier islands in the southern portionof Onslow Bay are located in a broad, shallow, high-energyshelf environment. Onslow Bay is a microtidal environment,with a mean tidal range of about 1 m. Based on four years ofwave gage data obtained at Wrightsville Beach during 1971-1975 (Jarrett, 1977), the mean significant wave height in thisarea is 0.78 m, with a corresponding period of 7.88 s. Thedominant direction of wave approach is from the northeastduring the winter months, and from the southeast during thesummer. Typically, storm waves approach from the northeast,but the area is also subject to episodic storm waveevents from the east and south during the passage of tropicaland extratropical cyclones.The introduction of new sediment to Onslow Bay is negligibledue to: 1) no fluvial input (coarse sediments aretrapped in the estuarine system); and 2) minimal sedimentexchange between adjacent shelf embayments (Cleary andPilkey, 1968; Blackwelder et al., 1982). Milliman et al.(1972) classified the Onslow Bay shelf sediment cover asresidual (derived from the erosion of underlying sedimentsand rocks). The major sources of sediment for the shorefaceand inner shelf are shoreface bypassing of unconsolidated,ancient sediments and bioerosion of marine hardgrounds onthe inner shelf. Bioerosion of the hardgrounds produces aresidual mix of sediment ranging from gravels to lime mud.These residual sediments are mixed with outcrop-associated,relatively fresh invertebrate fragments, including small coralsand shell material.The morphology of the shoreface from WrightsvilleBeach to southern Topsail Island is dominated by shore-normalrippled scour depressions (a genetic term used by Cacchioneet al., [1984] to describe similar features in othershelf environments). The depressions develop just outsidethe fair-weather surf zone at 3-4 m water depth, and extendto the base of the shoreface at about 10 m depth. On a sidescan-sonar mosaic (Figure 3), the rippled scour depressionsare defined by areas of high acoustic reflectivity (light-coloredareas). The depressions are floored with very coarseshell hash and quartz gravel, and on the upper shoreface arescoured up to 1 m below the surrounding areas of fine sand.Long, straight-crested, symmetric megaripples floor thedepressions. The depressions terminate and the shore-normalmorphologic fabric becomes shore-oblique at the base of theshoreface, due to a series of east- to northeast-trending, relictridges with 1-2 m of relief. Other features shown in Figure 3are highlighted in Figure 4.Recent surveys off Topsail Island show a similar, butless well-developed cross-shore morphology of rippled scourdepressions. This is likely due in part to the greater abundanceof rock outcrops that control the shoreface profileshape. The shoreface sediment volume, as suggested by ourseismic data, also appears to be smaller than that at WrightsvilleBeach. Thus, this area encompasses a fairly wide rangeof shoreface characteristics and morphologic types.RECENT HURRICANE IMPACTSAs this field trip has highlighted, Onslow Bay has beenrecently impacted by hurricanes Bertha and Fran. Thesestorms have provided a unique opportunity to investigatestorm processes in a well-documented and well-studiedshoreface system. Preliminary results from our post-stormstudies, which are literally works in progress, are summarizedbelow.Repeated sidescan-sonar surveys off Wrightsville Beachindicate that the gross morphology of the shoreface and innershelf did not change appreciably over the 38-month periodbetween the 1992 and 1995 sidescan-sonar surveysdescribed above. A sedimentary facies map presented byThieler et al. (1995) based on the 1992 data is remarkablysimilar to the digital sidescan mosaics (just completed and asyet unpublished) produced in 1994 and 1995. This suggeststhat the interannual variability of the shoreface sedimentcover is rather small; "typical" storms do not result in fundamentalchanges in the sedimentary fabric.Hurricane Bertha appears to have been an event that lefta minor, although distinct sedimentologic imprint. We havenoted some moderate changes in the textural characteristicsof the shoreface sediment cover, as well as a distinct local- toregional-scale storm bed. Hurricane Fran, however, was ageologically more important event. Post-Fran sidescan surveys,particularly off Topsail Island, indicate substantialreworking and redistribution of the shoreface and inner shelf22

SHOREFACE PROCESSES IN ONSLOW BAYFigure 3. Sidescan-sonar image of the Wrightsville Beach shoreface and inner shelf obtained in March 1994, showing locations ofseveral shore-normal to shore-oblique rippled scour depressions (RSDs), as well as features shown in Figure 4A-C. Areas of highacoustic backscatter are shown as light to white colored (generally medium sand and coarse, including rock outcrops); low acousticbackscatter is dark to black (finer than medium sand, and mud).23

E. Robert ThielerFigure 4. Enlarged view of features identified in Figure 3. A) A mud-filled channel outcropping on the inner shelf is shown here asa band of low backscatter (dark) sediment bounded by areas of slightly higher backscatter. This is a mid-Holocene tidal creek beingexhumed by shoreface erosion. B) The rippled scour depressions on the shoreface and inner shelf typically have a sharp contactbetween coarse (high backscatter- white) and fine (low backscatter-dark) sediment. The southern contacts, however, appear more“feathered” or “wispy.” C) Low-relief limestone outcrops in the study area is characterized by a somewhat diffuse pattern of highbackscatter on the sidescan image. Rock scarps, however, are identifiable as curvilinear narrow bands of high acoustic backscatter.Several are identifiable in the southeastern portion of Figure 3.24

SHOREFACE PROCESSES IN ONSLOW BAYsediment cover. Many rippled scour depressions have beencovered completely by fine to medium sand, and there isabundant evidence for strong, seaward-directed currents.These currents deposited large, lobe-like sand bodies at thebase of the shoreface. This is particularly apparent wherestorm overwash did not occur; large dunes appear to haveintensified downwelling currents and seaward sedimenttransport.A number of studies (e.g., Hayes, 1967; Aigner, 1985)have documented the presence of tempestites (graded stormlayers) in shelf sediments. The areal extent of individualevent layers, however, is not well known. Hurricane Berthaproduced a distinct and mappable tempestite layer on theinner shelf. While we do not have post-Fran vibracores, thesidescan-sonar data suggests that the post-Bertha imprintwas probably replaced by a more regional and deeper (in thesedimentary section) event layer.Recent studies of the shoreface geologic framework(e.g., Riggs, et al., 1995; Thieler et al., 1995) and age- mixingof the shell fraction on modern beaches (Wehmiller etal., 1995) indicate that shoreface bypassing, particularly thataccomplished during storms, is an important process on theNorth Carolina coast. Both hurricanes have created new orfresh exposures of ancient sediments and rocks on the shorefaceand inner shelf. Marine hardbottoms produce much ofthe residual sediment cover in Onslow Bay as they are biologicallyand physically eroded. Many of the areas in oursurveys have been buried, eroded or otherwise ”modified" asa sediment source. Our previous work has also identified theimportance of sedimentary texture and geologic control onshoreface sediment transport pathways. Have previouslyidentified transport pathways been opened or closed? If so,what is the nature of that change? Will these events have anysignificant impact on the longer-term evolution of the shoreface?These questions await further study.REFERENCESAigner, T., 1985. Storm depositional systems: Dynamic stratigraphyin modern and ancient shallow- marine sequences. LectureNotes in Earth Sciences Number 3. Springer-Verlag, New York,174 pp.Bijker, E.W., 1967. Some considerations about scales for coastalmodels with moveable beds. Delft Hydraulics Research LaboratoryTechnical Report No.50. Delft, The Netherlands.Blackwelder, B.W., Macintyre, I.G., and Pilkey, O.H., 1982. Geologyof the continental shelf, Onslow Bay, North Carolina, asrevealed by submarine outcrops. American Association ofPetroleum Geologists Bulletin, 66: 44-56.Bruun, P., 1954. Coast erosion and development of beach profiles.U.S. Army Corps of Engineers, Beach Erosion Board, TechnicalMemorandum 44.Bruun, P., 1962. Sea-Ievel rise as a cause of shore erosion. Proceedings,Journal of the Waterways and Harbors Division. ASCE,New York, 88: 117- 130.Cacchione, D.A. and Drake, D.E., 1982. Measurements of stormgenerated bottom stresses on the continental shelf, Journal ofGeophysical Research, 87: 1952-1960.Cacchione, D.A., Drake, D.E., Ferreira, J. T., and Tate, G.B., 1994.Bottom stress estimates and sand transport on the northern Californiainner continental shelf. In: J.H. Trowbridge and A.R:M.Nowell (Editors), Continental Shelf Research (Special volume,Sediment Transport Events on the Shelf and Slope -STRESS),14: 1271-1289.Cacchione, D.A., Grant, W.D., Drake, D.E., and Glenn, S., 1987.Storm-dominated bottom boundary layer dynamics on thenorthern California continental shelf: Measurements and predictions.Journal of Geophysical Research, 92: 1817-1827.Cacchione, D.A., Drake, D.E., Grant, W.D., and Tate, G.B. 1984.Rippled scour depressions on the inner continental shelf offcentral California. Journal of Sedimentary Petrology, 54: 1280-1291.Cleary, W.J., and Pilkey, O.H., 1968. Sedimentation in Onslow Bay.In: Guidebook for field excursions, Geological Society ofAmerica, Southeastern Section, Southeastern Geology SpecialPublication 1, Durham, North Carolina, 17 pp.Curray, J.R., 1969. Shore zone sand bodies - Barriers, cheniers, andbeach ridges. In: D.J. Stanley {Editor), The New Concepts ofContinental Margin Sedimentation. American Geological Institute,Washington, p. JCII-1-JCII-18.Dean, R.G., 1977. Equilibrium beach profiles: U.S. Atlantic andGulf Coasts. Department of Civil Engineering, University ofDelaware, Newark, Ocean Engineering Technical Report 12, 46pp.Dean, R.G., 1983. Principles of beach nourishment. In: P.D. Komar{Editor), Handbook of Coastal Processes and Erosion: CRCPress, Boca Raton, pp.217-231.Dean, R.G., 1987. Coastal sediment processes: Toward engineeringsolutions. Proceedings, Coastal Sediments '87, New York,ASCE, pp. 1- 24.Dean, R.G., 1991. 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E. Robert ThielerHanson, H., and Kraus, N.C., 1989. Genesis: Generalized Modelfor Simulating Shoreline Change. U.S. Army Corps of Engineers,CERC, Technical Report CERC-89-19, Vicksburg, Mississippi,185 p.Hayden, B.P., Felder, W., Fisher, J., Resio, D., Vincent, L., andDolan, R., 1975. Systematic variations in inshore bathymetry.University of Virginia, Department of Environmental Sciences,Charlottesville, Virginia, Technical Report 10.Hayes, M.O., 1967. Hurricanes as geologic agents: Case studies ofHurricane Carla, 1961 and Cindy, 1963. University of TexasBureau of Economic Geology Rept. Inv. No.61 , 56 pp.Hine,A.C., and Snyder, S.W., 1985. Coastallithosome preservation:Evidence from the shoreface and inner continental shelf offBogue Banks, North Carolina: Marine Geology, 63: 307-330.Jarrett, J. T., 1977. Sediment budget analysis, Wrightsville Beach toKure Beach, North Carolina. Proceedings, Coastal Sediments'77. ASCE, New York, pp. 986-1005.Jonsson, I.G., 1966. 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D., 1976, Sediment Transport in theCoastal Environment: Cambridge, Massachusetts, MassachusettsInstitute of Technology, Dept. of Civil Engineering Report209, 105 p.Milliman, J.D., Pilkey, O.H., and Ross, D.A., 1972. Sediments ofthe continental margin off the eastern United States. GeologicalSociety of America Bulletin, 83: 1315-1334.Moore, B.D., 1982. Beach profile evolution in response to changesin water level and wave height, M.S. thesis, University of Delaware,Newark, Delaware.Niedoroda, A.W., Swift, D.J.P., and Hopkins, T.S., 1985. The shoreface.In: R.A. Davis {Editor), Coastal Sedimentary Environments.Springer- Verlag, New York, pp. 533-624.Nielsen, P., 1979. Some basic concepts of wave sediment transport.ISVA, Technical University of Denmark Serial Paper No.20.Nittrouer, C.A., and Wright, L.D., 1994. Transport of particlesacross continental shelves. Reviews of Geophysics, 32: 85-113.Nummedal, D., 1991. Shallow marine storm sedimentation -theoceanographic perspective. In: G. Einsele, W. Ricken, and A.Seilacher {Editors), Cycles and Events in Stratigraphy.Springer-Verlag, New York, pp. 227-248.Pilkey, O.H., 1993. Can we predict the behavior of sand in a timeand volume framework of use to humans? {Editorial), Journalof Coastal Research, 9: .Pilkey, O.H., Young, R.S., Riggs, S.R., Smith, A.W., Wu, H., andPilkey, W.D., 1993. The concept of shoreface profile of equilibrium:A critical review. Journal of Coastal Research, 9: 255-278.Riggs, S.R., Cleary, W.J., and Snyder, S.W., 1995. Influence ofinherited geologic framework on barrier shoreface morphologyand dynamics. Marine Geology, 126: 213-234.Rosen, P., 1978. A regional test of the Bruun Rule of shoreline erosion.Marine Geol., 26: 7-16.Shi, N.C., and Larsen, L.H., 1984. Reverse sediment transportinduced by amplitude modulated waves. Marine Geology, 54:181-200.Sternberg, R.W., 1972. Predicting initial motion and bed load transportof sediment particles in the shallow marine environment.In: D.J.P. Swift, D.B. Duane, and O.H. Pilkey {Editors), ShelfSediment Transport: Process and Pattern. Dowden, Hutchinsonand Ross, Stroudsburg, pp. 61-82.Sternberg, R.W., and Larsen, L.H., 1975. Threshold of sedimentmovement by open ocean waves: Observations. Deep-SeaResearch, 22: 299-309.Sverdrup, H.U., Johnson, M.W., and Fleming, R.H., 1942. TheOceans: Their Physics, Chemistry, and General Biology. Prentice-Hall,New York, 1060 pp.Swift, D.J.P. 1976. Coastal sedimentation. In D.J. Stanley and D.J.P.Swift {Editors), Marine Sediment Transport and EnvironmentalManagement. John Wiley and Sons, New York, pp. 255-309.Thieler, E.R., Brill, A.L., Cleary, W.J., Hobbs, C.H., and Gammisch,R.A., 1995. Geology of The Wrightsville Beach, NorthCarolina shoreface: Implications for the concept of shorefaceprofile of equilibrium. Marine Geology, 126: 271-287.Vincent, C.E., Young, R.A., and Swift, D.J.P., 1982. On the relationshipbetween bedload and suspended sand transport on theinner shelf, long Island, New York. Journal of GeophysicalResearch, 87: 4163-4170.Wehmiller, J.F., York, L.L., and Bart, M.L, 1995. Amino acid racemizationgeochronology of reworked Quaternary mollusks onU.S. Atlantic coast beaches; implications for chronostratigraphy,taphonomy, and coastal sediment transport. Marine Geology,124: 303-337.Wiberg, P.L., and Smith, J.D., 1983. A comparison of field data andtheoretical models for wave-current interactions at the bed onthe continental shelf. Continental Shelf Research, 2: 147-162.Wright, L.D., 1987. Shelf-surfzone coupling: Diabathic shorefacetransport. Proceedings, Coastal Sediments '87. ASCE, NewYork, pp. 25-40.Wright, L.D., 1989. Benthic boundary layers of estuarine andcoastal environments. Reviews In Aquatic Sciences, 1: 75-95.Wright, L.D., 1993. Micromorphodynamics of the inner continentalshelf: A Middle Atlantic Bight case study. Journal of CoastalResearch, Special Issue 15, p. 93-123.Wright, L. D., Boon, J. D., Green, M. 0., and List, J. 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SHOREFACE PROCESSES IN ONSLOW BAYdiction of beach and surf zone morphodynamics: equilibria,rates of change and frequency response. Proceedings, CoastalEngineering Conference, 19th. ASCE, New York, pp.2150-2164.Zeidler, R., 1982. Profile of equilibrium. In: M.L. Schwartz (Editor),The Encyclopedia of Beaches and Coastal Environments.Hutchinson Ross, Stroudsburg, pp. 660-661.27

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1982 Annual MeetingPages 29 - 40MORPHOLOGY AND DYNAMICS OF BARRIER AND HEADLAND SHOREFACES IN ONSLOW BAY, NORTHCAROLINAStanley R. Riggs 1 , William J. Cleary 2 and Stephen W. Snyder 31 Department of Geology East Carolina UniversityGreenville, NC 27858Tele: 919-328-6360; Fax: 919-328-43912 Department of Earth SciencesUniversity of North Carolina at WilmingtonWilmington, NC 28403-3297Tele: 910-395-3498; Fax: .910-350-40663 Department of Marine, Earth, and Atmospheric SciencesNorth Carolina State UniversityRaleigh, NC 27695Tele: 919-515-7912; Fax: 919-515-7802ABSTRACTPassive margin coastlines with limited sand supplies,such as much of the U.S. Atlantic margin, are significantlyinfluenced by the geologic framework of older stratigraphicunits that occur beneath and seaward of the shoreface. ManyU.S. east coast barrier islands are perched barriers in whichthe underlying, pre-modern sediments determine the morphologyof the shoreface and strongly influence modernbeach dynamics and composition. Perched barriers consist ofvariable layers of beach sand on top of older, eroding stratigraphicunits with highly variable compositions and geometries.Along many parts of the coastal system,stratigraphically-controlled bathymetric features on the innershelf modify waves and currents and thereby affect patternsof sediment erosion, transport, and deposition on the adjacentshoreface. It is essential to understand this geologicframework before attempting to model the large-scale behaviorof these types of coastal systems.In North Carolina, most shoreline features are controlledby the pre-Holocene stratigraphic framework of the shoreface;the beaches are perched on top of pre-existing Pleistocene,Tertiary, and Cretaceous sediments. The surficialgeology of the coastal zone is subdivided into two distinctprovinces resulting in different stratigraphic controls of theshoreface. North of Cape Lookout the geological frameworkconsists of a Quaternary sequence that fills a regional depositionalbasin called the Albemarle Embayment. The coastalzone south of Cape Lookout is dominated by Tertiary andCretaceous units that crop out across the coastal plain andcontinental shelf, with very thin Quaternary units onlylocally preserved. Superimposed upon this regional stratigraphyis an ancient drainage system resulting in a series of fluvialvalleys filled with younger coastal sediments separatedby large interfluve areas of older stratigraphic units. Thisresults in a coastal system in which the shoreface is eithernonheadland or headland dominated, respectively. Headlanddominated shorefaces are further divided into subaerial andsubmarine categories. Nonheadland dominated shorefacesare further divided into those influenced primarily by transgressiveor regressive processes, or channel-dominated depositionalprocesses (i.e., inlet migration or stream valley fill).Examples of each of these six types of shorefaces are presentedto demonstrate the control that the geologic frameworkexhibits on shoreface morphologies and processes.INTRODUCTIONShoreface Profile of EquilibriumFenneman (1902) originally defined a shoreface profileof equilibrium as a profile that "the water would ultimatelyimpart, if allowed to carry its work to completion." Recently,many other workers have expanded upon the definition of theshoreface profile of equilibrium including the following:Schwartz (1982): "a long-term profile of ocean bed producedby a particular wave climate and type of coastal sediment";Dean (1983): "an idealization of conditions which occur innature for particular sediment characteristics and steadywave conditions"; and Larson (1991): "a beach of specificgrain size, if exposed to constant forcing conditions, normallyassumed to be short-period breaking waves, willdevelop a profile shape that displays no net change in time:'Bruun (1962) developed a simple model to characterizethe profile of equilibrium that is assumed to exist for allshorefaces, and which has become the basis for most modelsin the design of coastal engineering projects. However, Kriebelet al. (1991) argued that "a beach profile in true equilibriumnever exists in nature because nearshore water levels,waves, and currents are constantly changing."Pilkey et al. (1993) believe that there are many "shorefaceprofiles of equilibrium" and the model as developed by29

Stanley R. Riggs, William J. Cleary and Stephen W. SnyderBruun (1962) is too simplistic and based upon too manyassumptions. Pilkey et al. (1993) attacked the use of such asimplistic model on the grounds that four basic assumptionsare erroneous: 1) sediment movement is only driven byincoming wave orbitals acting on a sandy shoreface; 2) aquantifiable closure depth exists with no net transport of sedimentto and from the shoreface; 3) the shoreface is sand-richand the underlying or offshore geology do not influence theprofile shape; and 4) if a shoreface is sand-rich, thesmoothed profile described by the equilibrium profile equationmust provide a useful approximation of the real shorefaceshape.The present paper focuses on assumption three of Pilkeyet al. (1993). Our objective is to demonstrate the role of thegeologic framework in determining coastal barrier morphologyand shoreface dynamics for the North Carolina coastalsystem. The conclusions seem self evident, but it has becomeobvious that the concept of shore face profile of equilibriumhas been over simplified, is poorly understood, and is somewhatcontroversial. We believe that it is imperative to incorporatethe geologic framework into all models concerningthe large-scale behavior of any coastal system.Perched Beach SystemsMany beaches do not achieve profiles of equilibriumdue to eustatic sea-level fluctuation, lack of adequate sedimentsupplies, or the variable influence of the geologicframework upon which the beach is superimposed. Obviousexamples of the latter are the active margin coastlines of theU.S. Pacific, which are dominated by wave-cut platformsand associated strandplain beaches. These shoreface profilesare unquestionably dictated by the characteristics of theeroding headlands. In a similar but less dramatic way, passivemargin coastlines with limited sand supplies, such as theU.S. east coast, commonly have barrier islands perched uponpre-modern stratigraphic units that occur beneath and seawardof the shoreface. These stratigraphic units control themorphology of the shoreface and strongly influence modernbeach dynamics, sediment composition, and sediment fluxes(Riggs and O'Connor, 1974; Pearson, 1979; Riggs, 1979;Crowson, 1980).Perched barriers will not develop a profile of equilibrium,as previously defined by Bruun (1962), for two reasons.First, perched barriers consist of thin and variablelayers of surficial beach sands on top of older, eroding, stratigraphicunits with highly variable compositions and geometries.Depending upon the composition and geometry, thisunderlying platform will act as a submarine headland influencingthe shoreface dynamics and resulting profiles. Forexample, if these submarine headlands are composed ofcompact muds, limestones, or sandstones there will be agreater effect upon both the planform of barriers and morphologyof the shoreface and inner shelf than shorefacescomposed of unconsolidated sands and soft muds. Second,along many parts of the coastal system, bathymetric shoalfeatures occur on the inner shelf. These features will modifyincoming wave and current energy affecting the patterns ofsediment erosion, transport, and deposition on the adjacentbeaches.Fisher (1967) described the mid-Atlantic coastal systemas a series of coastal compartments. Each compartment consistedof an eroding mainland beach at the northern end witha barrier spit extending southward and grading into a seriesof barrier islands fronting a major estuarine system. Fisher(in Swift, 1969) interpreted the entire North Carolina barrierisland system as a "southern spit" that formed off a singleeroding headland at Cape Henry, Virginia (Fig. 1). Fisher'sinterpretation was a good first approximation based totallyupon subaerial databases and studies. However, when theunderlying geologic framework is considered, there are awhole series of eroding headlands that occur along the NorthCarolina coast (Fig. 2). These underlying framework unitsgenerally occur either in the shallow subsurface or submergedbelow the shallow coastal waters.Classes of Inherited Geologic FrameworkSix general classes of shoreface systems are recognizedalong the Onslow Bay portion of the North Carolina coastand are based on differences in the geologic framework. Theclassification scheme is based upon the designation of headlandand nonheadland categories (Fig. 2). The headland featuresand associated valley fill segments represent thetopographic or bathymetric features of Pleistocene or olderunits of varying compositions that produce the regional controlsfor the North Carolina coastal system (Riggs, 1979;Snyder, 1982, 1994; Riggs et al., 1990, 1992; Snyder et al.,1994).A. Headland dominated shorefaces are those with morphologicalfeatures that rise above the active ravinement surfaceand are dominantly composed of semi-indurated to indurated,Pleistocene or older sediment units. Two subclassesare recognized.1. Subaerial headland shorefaces are characterized by awavecut cliff and platform that are actively being incised intoPleistocene or older sediments with a perched beach.2. Submarine headland shorefaces are submerged morphologicalfeatures that have been incorporated into the modernshoreface and upon which the barrier-estuarine system isperched. Older sediments crop out on the eroding shorefaceand commonly occur on the inner shelf as bathymetric highsseaward of the modern shoreface and thus modify incomingwaves.B. Non-headland dominated shorefaces are those withoutheadland associations and are dominated by Holocene processesand sediments. Four subclasses are recognized.3. Transgressive shorefaces are composed of compact30

MORPHOLOGY AND DYNAMICS OF BARRIER AND HEADLAND SHOREFACES IN ONSLOW BAY, NORTH CAROLINAFigure 1. Map of the North Carolina coastal zone showing geographic mentioned in the text, along with the location of the fourcoastal segments discussed in the text (boxed areas).Holocene peat and mud that extend from the estuaries, underthe barrier sands, and crop out within the surf zone and uppershoreface. The shoreface is often characterized by an irregulargeometry with discontinuous, highly scarped dune ridgesand abundant washover fans on the barrier island.4. Regressive shorefaces are composed of unconsolidatedHolocene sands and occur along barrier island stretches withadequate sediment supplies. They often form in associationwith headlands, cape structures, and inlets and exhibit concaveprogradational geometry with accretionary beach ridgeson the barrier island.5. Channel-dominated inlet-fill shorefaces are composedof unconsolidated Holocene sand and gravel sediments thathave back-filled old inlet systems. They have limited longshoreextent and form on barrier islands in response to inletsystems that actively open, migrate, and close.6. Channel-dominated valley-fill shorefaces are sectionsalong barrier islands that have historically been occupied bypaleofluvial drainage systems and are underlain by thickaccumulations of fluvial- estuarine channel-fill sediments inresponse to deglaciation and Holocene sea-level rise.The basic structural and stratigraphic characteristics ofany coastal complex significantly influence the resulting barrierisland morphology and shoreface dynamics, and thereforeprevent the concept of an equilibrium profile from beingrealized. We present examples of perched beaches associatedwith each of the six different shoreface classes defined above(Figs. 1 and 2). Although all examples in this paper are fromOnslow Bay, NC, we do not attempt to address and classifythe entire Onslow Bay ocean shoreline.31

Stanley R. Riggs, William J. Cleary and Stephen W. SnyderFigure 2. Map of the North Carolina coastal zone showing the major paleofluvial valleys and associated interfluve headland features.NORTH CAROLINA COASTAL SYSTEMStructural SettingThe shallow geology of the North Carolina coastal zonecan be subdivided into the geologically distinct northern andsouthern provinces. North of Cape Lookout (Fig. 1), thecoastal zone is characterized by a thick Quaternary sequence(50 to 70 m) that fills a regional depositional basin parallel toAlbemarle Sound and called the Albemarle Embayment(Ward and Strickland, 1985). South of the Cape LookoutHigh (Fig. 1), the coastal zone is dominated by Tertiary andCretaceous units. The older and more lithified, offlappingstratigraphic sequences wrap around the Carolina PlatformHigh, a major basement structural feature that occurs southof Cape Fear, and crop out across much of the continentalshelf in Onslow and Long Bays (Snyder, 1982; Riggs et al.,1990). These Tertiary and Cretaceous stratigraphic units,along with local, remnant Quaternary sediment units, form abasal platform with variable topography upon which many ofthe modern barriers in the southern province are perched.Influence Upon Beach SedimentsAncient sediment deposits have been vibracored undermany shoreface sands along the entire Atlantic and Gulfcoast. Marsh peats, tidal flat muds, fluvial sands and gravels,bay-fill sands and muds, flood- tide delta sands, and inlet-fillsands and gravels commonly occur below a thin veneer ofmodern shoreface sands that are generally

MORPHOLOGY AND DYNAMICS OF BARRIER AND HEADLAND SHOREFACES IN ONSLOW BAY, NORTH CAROLINAFigure 3. Oblique aerial photograph looking southwest fromKure Beach (A) to Fort Fisher (B), across the Onslow subaerialheadland (C), and to the Cape Fear River estuary (D). Thisphoto also shows the following features: (1) Pleistocenecoquina sandstone outcrop in the surf zone; (2) man-made rockrevetments to slow the rates of shoreline recession along thewave-cut cliff of Pleistocene friable humate quartzose sands;and (3) rapidly retreating shoreline associated with the channel-dominatedvalley fill shoreface.characteristics of barrier island beach sands is strong evidencethat relict sediments are being eroded from the shoreface(Moorefield, 1978; Pearson, 1979; Crowson, 1980;Pearson and Riggs, 1981; Cleary and Hosier, 1990). Supportfor the conclusion that relict and residual sediments areactively being eroded from the shoreface and deposited onthe beach includes the following.1. Sections of beach between Nags Head and the Virginialine (Fig. 1) contain abnormally high concentrations ofquartz and lithoclast gravel, which was" mined for constructionaggregate during historical times. These beach gravelsoccur in areas where seismic data demonstrate the -presenceof paleofluvial channels passing beneath the barrier andcropping out on the adjacent continental shelf (Riggs andO'Connor, 1974; Riggs, 1979; Eames, 1983; Riggs et al.,1992).2. The extinct fossil oyster Crassostrea gigantisima, andassociated Oligocene rock lithoclasts, occur in great abundanceon Onslow Beach and Topsail Island after storms(Crowson, 1980; Cleary and Hosier, 1987). The eroded gravelsare derived from the bioerosion of Oligocene hard bottomscarps that crop out on the inner shelf. These gravels are subsequentlytransported up the shoreface during high-energystorms and left on the beach in the same fashion as heavyminerals at the top of the swash zone of a storm beach.3. Overwash terraces on Masonboro Island contain abundantcobble-size coquina clasts and mollusk shells derived fromhardbottoms exposed on the adjacent inner shelf. Also, muchof the coarse- grained component of the beach sediment canbe attributed to the onshore transport of reworked and palimpsestsediments that mantle these hardbottoms. Stormreworking of the thin shoreface sediment cover and thedegraded character of underlying rock units appear to contributesignificant amounts of coarse material to the adjacentbeaches (Cleary et al., 1992, 1993).4. Black-stained oysters and other estuarine fossils are thedominant shell on many North Carolina beaches. Theseshells always produce pre- modern, Holocene ages whendated by carbon- 14 techniques (Pilkey et al., 1969; Wehmiller,1993).5. Mixed assemblages of Pleistocene age marine shells occurin great abundance on many of the North Carolina beachesanalyzed by amino-acid racemization dating techniques(Wehmiller, 1993).Subaerial Headland ShorefacesFort Fisher to Kure BeachNorth Carolina's only subaerial headlands occur oneither side of the Cape Fear River estuary and include portionsof Yaupon and Long Beaches (Griffin et al., 1977) tothe south, as well as the shoreline between Fort and KureBeach to the north. In the latter area, an extensive erodingsubaerial headland intersects the coastal zone without a barrierisland-estuarine system (Fig. 2). The coastal system consistsof a wave-cut platform incised into Oligocene throughPleistocene units of the mainland peninsula with a thin beachperched on top of the irregular geometry of the Pleistoceneunits (DuBar et al., 1974; Moorefield, 1978; Meisburger,1979; Cleary and Hosier, 1979; Snyder et al. 1994).Figure 3 shows the dramatic relationship between threedifferent geologic framework situations in the Fort Fisherarea and geometry of the shoreline and upper shoreface. Erosionresistant, lithified and cross-bedded coquina sandstoneforms a headland in the shoreline north of Fort Fisher (Fig.3). Friable humate and iron-cemented Pleistocene sandstone(Fig. 4) forms a 2 m high wave-cut cliff and terrace thatfronts the shoreline immediately south of the headland andseaward of the Civil War Fort Fisher. South of Fort Fisher isa nonheadland segment characterized by a channel-dominated,valley-fill shoreface (Fig. 3) underlain by 10 m ofmuddy estuarine sediments (Swain and Cleary, 1992). Theshape and evolution of the three different coastal compartmentsaround Fort Fisher is clearly related to the presenceand lithology of the outcropping and underlying Pleistocenegeologic framework.Moorefield (1978) mapped beach outcrops of Pleistocenecoquina north of Fort Fisher and their seaward extensionson the inner shelf. Our ongoing studies clearly showthat coquina and its associated lithologies form a series ofwidespread, irregular, bathymetrically high hardbottom featureswith >3 m of relief. This karstic mosaic includes oneextensive hardbottom area known as Sheephead Rock thatlies in 9 m of water with pedestal-like hardbottom featuresrising to within 2.5 m of the ocean surface. Diver observa-33

Stanley R. Riggs, William J. Cleary and Stephen W. SnyderFigure 4. Photograph looking north at the Fort Fisher (A) andthe Pleistocene coquina subaerial headland (B). This photoalso shows the: (C) Pleistocene friable humate- and ironcementedsandstone that forms a two meter high wave-cutcliff; (D) rock revetment built to protect the rapidly receedingshoreline and the Civil War Fort Fisher; and (E) the strandplainbeach. Photo was taken in 1977 and the shoreline hadalready receeded behind the rock revetment; today the shorelinehas receeded an additional 20 to 25 meters.tions and cores suggest that the sediment cover is bothpatchy and very thin across much of this region and in manyareas is totally lacking.The extensive series of coquina outcrops on the innershelf act as barriers that could significantly affect the refractionof wave energy, as well as the movement of sand acrossthis shoreface. Moorefield (1978) believes that sand fromboth the rapidly eroding beach at Fort Fisher and littoraldrift, are transported seaward of the rock barrier duringstorms and prevented from returning to the beach duringsubsequent low energy periods. The result of this process is anet sediment deficiency in which the rapidly retreating bluffshoreline is consuming the historic Fort. A variance wasreceived from the State to build anew rock revetment to protectFort Fisher; the present bulkhead structure was completedin 1995.Submarine Headland Shorefaces and BathymetricHighsOnslow Beach to Topsail IslandThe New River Inlet coastal area (Fig. 1) is a submarineheadland, which forms a small seaward bulge in the coastlineof central Onslow Bay (Fig. 2). This shoreline protrusion isproduced by the Oligocene Silverdale Formation, an induratedmoldic limestone and calcareous-cemented quartzsandstone unit. The Silverdale Formation crops out at orslightly below sea level in the mouth of the New River estu-Figure 5. Schematic diagram looking southwest along the hardbottom scarps of the Oligocene Silverdale Formation that crop out onthe inner continental shelf off of Topsail Island. The scarps are parallel to and face the beach, up to 5 m high with major overhangs,and dip gently seaward. Extensive bioerosion and wave processes produce “new” gravel sediment which is transported directly tothe beach during storms. The sandy limestone crops out at sea level in the estuary behind the barrier; however, it is not known howthe barrier is perched on top of this rock unit. Figure is modified from Crowson (1980).34

MORPHOLOGY AND DYNAMICS OF BARRIER AND HEADLAND SHOREFACES IN ONSLOW BAY, NORTH CAROLINAFigure 6. Map showing the outcrop pattern of the Oligocenerock scarps on the inner shelf formed by the Onslow submarineheadland. In addition, the map shows the lowstand channelcut by the New River and the area of intersection where thetopographically high rock features intersect the barrierislands. Notice the similarity of orientation patterns of boththe estuaries and older Pleistocene beach ridges.ary. It occurs extensively on dredge spoil islands of the IntracoastalWaterway behind Topsail Island and Onslow Beach,and forms a series of bathymetric ridges on the inner shelf oneither side of New River Inlet (Crowson, 1980). Crowsonmapped these prominent submarine rock features as a seriesof ridges that occur seaward of the lower shoreface, havesteep landward-facing scarps with smooth surfaces that dipgently away from the beach, and have up to 5 m of reliefabove the surrounding ravinement surface (Fig. 5). Theridges rise locally to about 5 m below MSL, higher than theelevation of the lower shoreface, which is probably highenough to cause major refraction of storm waves and currentsand possibly affect the patterns of erosion and depositionon the adjacent beaches.The ridges are oriented at acute angles to the beach andintersect the shoreface on Topsail Island and Onslow Beach(Fig. 6). Core drilling by Cleary and Hosier (1987) demonstratedthat the rock ridges continue under Onslow Beachand into the back- barrier estuarine system (Figs. 7 and 8).Similar limestone ridges pass beneath Topsail Island and intothe back-barrier estuarine system (Clark et al., 1986) wherethe rock structures appear to be related to the occurrence andorientation of Pleistocene barrier island systems (Fig. 6).The Oligocene submarine headland appears to subdividethese two barriers into coastal compartments that have differentorientations and shoreface dynamics. Figures 9 and 10show the changes in shoreline geometry that coincides withthe intersection of the Oligocene rock ridges. The northernsegment of Onslow Beach is characterized by a cuspateshoreline geometry with wide beaches, a recurved accretionarybeach ridge, a nearly continuous high dune ridge, andshoreline accretion rates that average 2 m/yr (Cleary andHosier, 1987). In contrast, the southern segment is characterizedby a narrow shoreface with abundant rock gravel on thebeach, a single discontinuous scarped foredune ridge, presenceof major washover terraces, and current erosion rates upFigure 7. Geologic cross-section along Onslow Beach based upon continuous split-spoon drill holes located in Figure 8. This sectionshows two Oligocene rock ridges that rise approximately 5 m below sea level and pass under Onslow Beach.35

Stanley R. Riggs, William J. Cleary and Stephen W. SnyderFigure 8. Map of the Onslow Beach showing the 1) location ofdrill holes used in Figure 7, 2) location (middle arrow) ofcoastal inversion that takes place where the Oligocene rockridges intersect the island, 3) plot of average annual rates ofshoreline recession from Benton et al. (1993) along OnslowBeach. The southern portion of the island is experiencingsevere shoreline recession whereas the northern pint is experiencingaccretion and dune ridge development. The area ofmost severe erosion adjacent to New River Inlet is largely adirect response to inlet modification.to 6 m/yr.Crowson (1980) believes that active bioerosion of therock scarps represent a major source and supply of 'new sediment'to the adjacent beaches (Fig. 5). Abundant gravel, upto boulder-size grains, is derived from the rock scarps andlower shoreface and delivered to the beach during stormswhere it is rapidly broken down to sand-sized grains in thesurf zone.Figure 9. Oblique aerial photograph looking northeast fromNew River Inlet to Onslow Beach (A). The approximate locationand orientation of the submerged Oligocene rock ridgesare indicated on the photo (B); notice the major change orientationof Onslow Beach northeast of the intersection of theseridges with the barrier island (C).Non-Headland Dominated ShorefacesTransgressive ShorefacesLarge segments of the NC barrier islands are underlainby deposits of estuarine peat and mud, often with in situ treestumps. These semi-indurated, back-barrier sediments cropout in the surf zone along major portions of Onslow Bay barrierislands, particularly during winter storms when much ofthe sand is transported off the beach and is stored in offshorebars. These fossil estuarine units were overrun by barrierisland systems as they migrated upward and landward inresponse to the general Holocene transgression. Obviously,these sediments have very different compositions, densities,cohesiveness, and resistance to erosion and transport thannormal beach sands. Consequently, their occurrence in theshoreface will affect the beach width, shoreface profile, andrates of shoreline recession.Discontinuous zones of 1,200 to 1,700 year old peathave been mapped along major portions of both MasonboroIsland (Fig. 1) .At the northeast end of Topsail Island, a 0.5m thick peat crops out periodically in the surf zone and canbe traced laterally around New River Inlet to a modern backbarriersalt marsh. Underlying the peat is a compact grayclay of unknown thickness. Storm erosion produces largeboulders (up to 0.7 m) of peat and clay, along with Oligocenerock fragments from the offshore scarps; these gravels representa significant input of 'new' post-storm beach sediment.Regressive ShorefacesOnly local and relatively small segments of the NorthCarolina shoreline are presently characterized by regressiveshoreface conditions. These areas generally occur on theflanks of headlands and represent temporary episodes ofcoastal progradation that usually alternate with episodes oflonger-term truncation as the headland recedes. However,during episodes of regression, these shorefaces are relativelystable, are characterized by progradational geometries, beachridge accretion, dune ridge development, and have the potentialfor approximating the idealized "profile of equilibrium".In Onslow Bay, Cape Lookout, Shackelford Banks, and BaldHead Island contain local and often temporary examples ofthis type of shoreface system (Fig. 1).Shoreface regression also takes place on a small scale onbeaches adjacent to some inlets. This process depends uponthe individual inlet processes and will develop only if there isan adequate sand supply. The result is the progradation of theshoreface adjacent to the inlet to produce the classic 'drumstick'barrier of Hayes (1976). The drumstick portion of thebarrier is characterized by wide beaches, continuous duneridges, multiple recurved accretionary beach ridges, andshoreline accretion. The northeastern end of Onslow Beach,adjacent to Browns Inlet, displays these characteristics(Cleary and Hosier, 1987). Similar inlet influenced barriersegments include Figure Eight Island downdrift of Rich Inlet36

MORPHOLOGY AND DYNAMICS OF BARRIER AND HEADLAND SHOREFACES IN ONSLOW BAY, NORTH CAROLINAFigure 10. Oblique aerial photograph looking southwest fromthe southern end of Onslow Beach (A), across New River Inlet(B), and to Topsail Island (C). The approximate location andorientation of the submerged Oligocene rock ridges are indicatedon the photo (D); notice the major change in orientationof Topsail Island southwest of the intersection of these ridgeswith the barrier island (F).(Fig. 1).During prior small-scale, sea-level fluctuations thatoccurred during the overall Holocene transgression, conditionsof shoreface regression were common along the NCbarrier islands (Fisher, 1967). Under these conditions, shorefaceregression occurs in response to a minor drop in sealevel; if there is an adequate sand supply, the beach buildsseaward through the accretion of a series of beach ridges.The subsequent sea-level rise and transgression truncates thepreviously deposited set of beach ridges. Numerous barrierislands display a complex system of multiple beach ridges,which include Bogue Banks, Buxton Woods, and KittyHawk Woods. The central and western portions of BogueBanks consist of sets of prograding beach ridge structuresthat probably formed in this way (Steel, 1980).Channel-Dominated, Inlet-Fill ShorefacesFisher (1962) mapped the spatial and temporal distributionof historic inlets along the North Carolina coast north ofCape Lookout from aerial photographs. An estimation basedupon Fisher's map suggests that about 50% of this coastalsystem has been occupied by inlets during the historic pastwith >78% having been occupied by inlets based upon thepresence of old flood-tide deltas landward of the barriers.These segments of the barriers are underlain by thick accumulationsof inlet fill sands and gravels (Cleary and Hosier,1979; Eames 1983; Hine and Snyder, 1985; Riggs et al.,1992).Today, the entire North Carolina coast north of CapeLookout (Fig. 1) contains only three major inlets (Oregon,Hatteras, and Ocracoke) and one minor inlet (Drum).Whereas, the area south of Cape Lookout (Fig. 1) representsa very different coastal compartment that is dominated by theOnslow submarine headland (Fig. 2). Thirteen modern inletsoccur within the coastal compartment between Cape Lookoutand Cape Fear.The eight modern inlets along the 76 km betweenOnslow Beach and Carolina Beach in the southern portion ofFigure 11. Map showing the position and movement of historical inlet zones on Shell, Figure Eight, and Coke Islands, North Carolina.Notice that the majority of these barriers have been impacted by historic inlets; vibracoring on the islands demonstrates that100% of these islands consist of inlet channel backfill sediments. Figure is modified from Hosier and Cleary (1977) and Brooks andCleary (1989).37

Stanley R. Riggs, William J. Cleary and Stephen W. SnyderOnslow Bay are highly migratory (Cleary and Hosier, 1979).They conclude that over 70% of the barrier island lengthsouth of New River Inlet has inlet channel-dominated shorefacesas a result of inlets migrating along the island duringthe last several centuries. For example, most of the coastalarea of Shell, Figure Eight, and Coke Islands have historicallybeen occupied by inlets (Fig. 11) .The result is a shorefacesystem that is underlain by channel sands and gravelsthat formed in response to a series of rapidly migratingHolocene inlets. Steel (1980) drilled and mapped a minimumof eleven relict inlets in the Holocene record beneath BogueBanks.Channel-Dominated Valley-Fill ShorefacesPortions of barrier islands that are within the valleys ofmajor Piedmont drainage systems (Fig. 2) have shorefacesthat are characterized by complex sediment sequencesdeposited in ancient paleofluvial valleys (Pearson, 1979;Hine and Snyder, 1985; Riggs et al., 1992; Snyder et al.,1994). These drainage systems have repeatedly incisedthemselves into their valleys and associated valley fill duringeach Pleistocene glacial episode when sea-level occupiedlowstand positions. During the subsequent transgression,these large channel complexes were systematically backfilledwith a new vertical succession of fluvial and estuarinesediments. The upper estuarine sediments generally are composedof fine-grained, muddy sediments. As the shorefacerecedes in response to ongoing transgression, the portion thatis underlain by estuarine valley fill, erodes rapidly relative tothe adjacent headland dominated shorefaces (Fig. 2).CONCLUSIONSAlong continental margins with limited sand supplies,such as the U.S. Atlantic coast, the shoreface is not an infinitelythick pile of sand. Rather, it is a thin, variable, andtemporal accumulation of sand superimposed or perchedupon a pre-existing and highly dissected geologic framework.Holocene sea-level rise has produced a modern transgressivebarrier island, estuarine, and fluvial sequence ofcoastal sediments that are being deposited unconformablyover irregularly preserved remnants of pre-existing stratigraphicsequences consisting of many sediment and rockunits of variable ages, origins, and compositions. It is thecomplex variability in this underlying geologic framework,in consort with the physical dynamics of each specificcoastal system, that ultimately determines the 1) threedimensionalshoreface morphology, 2) composition and textureof beach sediments, and 3) shoreline recession rates.Based upon the pre-Modern geologic framework, thereare six general categories of shoreface systems that occuralong the Onslow Bay coast of North Carolina. Headlandsare morphological features that rise above the active ravinementsurface and are composed of semi-indurated to indurated,Pleistocene or older sediment units. 1. Subaerialheadlands are characterized by the active incisement of awavecut platform and cliff into Pleistocene or older stratigraphicunits with an associated perched beach. 2. Submarineheadlands are submerged morphological featurescomposed of Pleistocene or older stratigraphic units thathave been incorporated into the modern shoreface and uponwhich the barrier-estuarine system is perched. Ancient sedimentscrop out on the eroding shoreface and commonlyoccur on the inner shelf as bathymetric highs seaward of themodern shoreface and thus, modify incoming waves and currents.Nonheadland shorefaces are dominated by Holoceneprocesses and sediments and can be divided into four generalsubclasses. 3. Transgressive shorefaces are composed ofcompact peat and mud that extend from the modern estuaries,under the barrier sands, and crop out within the surf zoneand upper shoreface. The steep shoreface is often characterizedby an irregular geometry and the beach is dominated bydiscontinuous and highly scarped dune ridges with abundantwashover fans. 4. Regressive shorefaces are composed ofunconsolidated sand and occur along barrier segments thathave adequate sediment supplies and are often associatedwith inlets, headlands, and cape structures. The beaches aredominated by progradational geometries with accretionarybeach ridges. Channel-dominated shorefaces consist of twodistinctive types of systems. 5. Inlet-fi!l shorefaces are composedof unconsolidated sand and gravel sediments with clinoforminfill geometry that forms in response to activelymigrating inlets. 6. Va!ley-fi!l shorefaces have historicallybeen occupied by large paleofluvial drainage systems andare underlain by thick accumulations of fluvial- estuarinechannel-fill sediments in response to deglaciation andHolocene sea-Ievel rise. Valley-fill sediments are typicallycomposed of fine-grained sediment and therefore will erodedifferently than inlet- fill sands and gravels. The sedimentsderived from valley-fill erosion are often not compatible withthe dynamics of the adjacent beach systems. Consequently,valley-fill shorefaces are often characterized by lower slopesand sediment deficient beaches that are actively eroding.Thus, the basic structural, stratigraphic, and geomorphiccharacteristics of the pre-barrier land surface interacts in acomplex way with modern coastal processes to determinethe barrier beachmorphology and shoreface dynamics. Eachbarrier beach and shoreface are total products of their geologicheritage; the signature of their geologic history controlsand influences the present morphology, shoreface dynamics,and rates of shoreline recession. Consequently, the conceptof a common equilibrium profile for all shorefaces is neitherrealistic nor adequate when considering detailed processesalong any given coastal segment. It is imperative that societylearn to live with and manage our complex shorelines. Inorder to do this, we must understand the detailed geologicframework underlying the shoreface and the inner shelf, as38

MORPHOLOGY AND DYNAMICS OF BARRIER AND HEADLAND SHOREFACES IN ONSLOW BAY, NORTH CAROLINAwell as the physical dynamics operating within and uponregional segments of the shoreface system. Then we canrealistically begin to model the large-scale behavior ofcoastal systems.ACKNOWLEDGEMENTSThis research is in part a product of research supportedby the following grants: N.C. Office of Marine Affairs(1978-1979) to SRR; National Science Foundation OCE-8110907, OCE-8118164, and OCE- 8342777 (1981-1984) toSRR; U.S. Marine Corps at Camp Lejeune, N.C. (1985-1987) to WJC; NOAA- National Undersea Research Center/University of North Carolina at Wilmington (1992-1993) toWJC and SRR. Appreciation is expressed to the many studentsof the Geology Departments at East Carolina Universityand University of North Carolina at Wilmington whohave helped develop the level of understanding of NorthCarolina coastal systems that lead to this paper.REFERENCESBenton, S.B., Bellis, C.J., Overton, M.F., Fisher, J.S., and Hench,J.L., 1993, Long term average annual rates of shoreline change:methods report 1992 update: North Carolina Department ofEnvironment, Health, and Natural Resources, Division ofCoastal Management, Raleigh, NC, 27611, 16 p, 14 plates.Brooks, W.B., and Cleary, W.J., 1989, Variations in island morphologyand inlet history: Figure Eight Island, N.C.: GeologicalSociety of America, Abstracts with Programs, v. 21 , no.3, p. 6.Bruun, P., 1962, Sea-Ievel rise as a cause of storm erosion: Proceedingsof the American Society of Civil Engineers, Journal of theWaterways and Harbors Division, 88(WWI), p. 117-130.Clark, P., Cleary, W.J., and Laws, R.A., 1986, ALate Pleistocenebay-barrier system: Topsail Sound, North Carolina: GeologicalSociety of America, Abstracts with Programs, v. 18, no.3, p.215.Cleary, W.J., and Hosier, P.E., 1979, Coastal geomorphology, washoverhistory, and inlet zonation: Cape Lookout to Bird Island,North Carolina, in Leatherman, S.D., ed., Barrier islands fromthe Gulf of St. Lawrence to the Gulf of Mexico: AcademicPress, New York, p. 237-262.Cleary, W.J., and Hosier, P.E., 1987, Onslow Beach, North Carolina:morphology and stratigraphy: Proc., Coastal Sediments'87, American Society Civil Engineering, New Orleans, p.1745-1759.Cleary, W.J., and Hosier, P.E., 1990, Storm climate and inlet stabilizationeffects on a transgressive barrier, Masonboro Island,North Carolina: Geological Society of America, Abstracts withPrograms, v. 22, no.4, p. 7.Cleary, W.J., Theiler, E.R., and Riggs, S.R., 1992, A reconnaissancesurvey of shoreface sedimentation off a replenished barrier,Wrightsville Beach, N.C.: Geological Society of New Zealand,Misc. Pub. 65A, Abstracts for IGCP 274 International Symposiumon Diversity in Coastal Evolution in the Quaternary, 1992,Wellington, N.Z., p. 13.Cleary, W.J., Riggs, S.R., and Theiler, E.R., 1993, Barrier/lagoonand shoreface Holocene stratigraphy: Masonboro Island, N.C.:Geological Society of America, Abstracts with Programs, v. 25,no.4, p. 8.Crowson, R.A., 1980, Nearshore rock exposures and their relationshipto modern shelf sedimentation, Onslow Bay, North Carolina:Unpub. M.S. Thesis, Dept. of Geology, East CarolinaUniv., Greenville, 128 p.Dean, R.G., 1983, Principles of beach nourishment, in P. Komar,ed., Handbook of coastal processes and erosion: Boca Raton,Florida, CRC Press, p. 217-231.DuBar, J.R., Johnson, H.S., Thorn, B.G., and Hatchell, W.O., 1974,Neogene stratigraphy and morphology, south flank of the CapeFear Arch, North and South Carolina, in Oaks, R.Q., andDuBar, J.R., eds., Post Miocene stratigraphy, central and southernAtlantic Coastal Plain: logan, Utah, Utah State Univ. Press,p. 139-173.Eames, G.B., 1983, The late Quaternary seismic stratigraphy, lithostratigraphy,and geologic history of a shelf-barrier-estuarinesystem, Dare Co., North Carolina: Unpub. M.S. Thesis, Dept.of Geology, East Carolina Univ., Greenville, 196 p.Fenneman, N.M., 1902, Development of the profile of equilibriumof the subaqueous shore terrace: Journal of Geology, v. 10, p. 1-32.Fisher, J.J., 1962, Geomorphic expression of former inlets along theOuter Banks of North Carolina: Unpub. M.A. thesis, Universityof North Carolina, Chapel Hill, N.C., 120 p.Fisher, J.J., 1967, Development patterns of relict beach ridges,Outer Banks barrier chain: Unpub. Ph.D. dissert., University ofNorth Carolina, Chapel Hill, N.C., 225 p.Griffin, W. 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W., 1985, Coastallithosome preservation:evidence from the shoreface and inner continental shelfoff Bogue Banks, North Carolina: Marine Geology, v. 63, p.307-330.Hosier, P.E., and Cleary, W.J., 1977, Cyclic geomorphic patterns onan overwash dominated barrier island in southeastern NorthCarolina: Environmental Geology, v. 2, p. 23-31.Kriebel, D.l., Kraus, N.C., and larson, M., 1991 , Engineering methodsfor predicting beach profile response: Coastal Sediments'91, American Society of Civil Engineers, p. 557-571.Larson, M., 1991, Equilibrium profile of a beach with varying grainsize: Coastal Sediments '91 , American Society of Civil Engineers,p. 905-919.Meisburger, E.P., 1979, Reconnaissance geology of the inner continentalshelf, Cape Fear region, North Carolina: U.S. ArmyCorps of Engineers, Coastal Engineering Research CenterTech. Paper 79-3, 135 p.Moorefield, T.P., 1978, Geologic processes and history of the FortFisher coastal area, North Carolina: Unpub. M.S. Thesis, Dept.of Geology, East Carolina Univ., Greenville, 100 p.Niedoroda, A.W., Swift, D.J.P., Figueiredo, A.G., and Freeland,39

Stanley R. Riggs, William J. Cleary and Stephen W. SnyderG.L., 1985, Barrier island evolution, middle Atlantic shelf,USA: Part II evidence from the shelf floor: Marine Geology, v.63, p. 363-396.Pearson, D.K., 1979, Surface and shallow subsurface sedimentregime of the nearshore inner continental shelf, Nags Head andWilmington areas, North Carolina: Unpub. M.S. Thesis, Dept.of Geology, East Carolina Univ., Greenville, 120 p.Pearson, D.K., and Riggs, S.R., 1981, Relationship of surface sedimentson the lower forebeach and nearshore shelf to beachnourishmnet at Wrightsville Beach, North Carolina: Shore andBeach, v. 49, p. 26-31.Pilkey, O.H., Blackwelder, B.W., Doyle, L.J., Estes, E., and Terlecky,T.M., 1969, Aspects of carbonate sedimentation on theAtlantic Continental Shelf off the southeastern United States:Journal of Sedimentary Petrology, v. 39, p. 744;.768.Pilkey, O.H., Young, R.S., Riggs, S.R., Smith, A.W.S., Wu, H., andPilkey, W.D., 1993, The concept of shorefaceprofile of equilibrium:a critical review: Journal of Coastal Research, v. 9, p.255-278.Riggs, S.R., 1979, A geologic profile of the North Carolina coastalinnercontinental shelf system, in Langfelder, J., Ocean OutfallWastewater Disposal Feasibility and Planning: N.C. State Univ.Press, p. 90-113.Riggs, S.R., and O'Connor, M.P., 1974, Relict sediment deposits ina major transgressive coastal system: North Carolina Sea GrantPub. No. UNC- SG-74-04, 37 p.Riggs, S.R., Snyder, Stephen W., Snyder, Scott W., and Hine, A.C.,1990, Stratigraphic framework for cyclical deposition ofMiocene sediments in the Carolina Phosphogenic Province, inBurnett, W.C., and Riggs, S.R., eds., Neogene to Modern Phosphorites:Cambridge University Press, Cambridge, England,Phosphate Deposits of the World, vol. 3, chpt. 29, p. 381-395.Schwartz, M.L., (ed.), 1982, The encyclopedia of beaches andcoastal environments: Stroudsburg, Hutchinson Ross, 940p.Riggs, S.R., York, L.L., Wehmiller, J.F., and Snyder, StephenW., 1992, Depositional patterns resulting from high frequencyQuaternary sea-Ievel fluctuations in northeastern North Carolina,in Fletcher, C.H., and Wehmiller, J.F., eds., QuaternaryCoasts of the United States: SEPM (Society for SedimentaryGeology), Spec. Pub. 28, p. 141-154.Snyder, Stephen W., 1982, Seismic stratigraphy within the MioceneCarolina Phosphogenic Province: chronostratigraphy, paleotopographiccontrols, sea-Ievel cyclicity, Gulf Stream dynamics,and the resulting depositional framework: Unpub. M.S. thesis,University of North Carolina, Chapel Hill, N.C., 183 p.Snyder, Stephen W., 1994, Miocene sea-Ievel cyclicity: resolutionof amplitudes and frequency from the Carolina Platform:Unpub. Ph.D. dissertation, Univsity of South Florida, St.Petersburg, 688 p.Snyder, Stephen W., Hoffman, C.W., and Riggs, S.R., 1994, Seismicstratigraphic framework of the inner continental shelf:Mason Inlet to New Inlet, North Carolina: N.C. Geological Survey,Bull. No.97 , 61 p.Steel, G.A., 1980, Stratigraphy and depositional history of BogueBanks, North Carolina: Unpub. M.S. thesis, Duke University,Durham, NC, 201 p.Swain, K.W., and Cleary, W.J., 1992, Modification of a CoastalPlain/bar built estuary, southeastern, North Carolina: GeologicalSociety of America, Abstracts with Programs, v. 24, no.2, p.69.Swift, D.J.P., 1969, Inner shelf sedimentation: processes and products,in Stanley, D.J., ed., The New Concepts of ContfnentalMargin Sedimentation: Washington, D.C., American GeologicalInstitute, p. 4-1 to 4-46.Swift, D.J.P., 1976, Continental shelf sedimentation, in Stanley,D.J., and Swift, D.J.P., eds., Marine Sediment Transport andEnvironmental Management: New York, Wiley, p. 311-350.Ward, L.W., and Strickland, G.L., 1985, Outline of Tertiary stratigraphyand depositional history of the U.S. Atlantic coastalplain, in Poag, C.W., ed., Geologic Evolution of the UnitedStates Atlantic Margin: Van Nostrand Reinhold Co., New York,p.87-124.Wehmiller, J.F., 1993, Aminostratigraphic evidence for age-mixingof Quaternary mollusks on mid-Atlantic beaches: taphonomicand chronostratigraphic implications: Geological Society ofAmerica, Abstracts with Programs, v. 25, no.6, p. A-462.40

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 41 - 50INLET INDUCED SHORELINE CHANGES: CAPE LOOKOUT — CAPE FEARINTRODUCTIONWilliam J. ClearyDepartment of Earth Sciences andThe Center for Marine Science ResearchUniversity of North Carolina at WilmingtonWilmington, NC 28403-3297During the last several years there has been a renewedinterest in tidal inlet research, principally from the managementviewpoint. Tidal inlet systems are links between adjacentbarriers and act as corridors for exchanging water,nutrients, pollutants and sediment between estuaries and theopen ocean. During the yearly to decadal time scales, inletsplaya major role in the coastal sediment budget by retaininglarge volumes of sand impounded from the littoral system.The extent to which these systems interrupt the along shoretransport and store sand depends largely upon the local waveclimate and the tidal prism.In the inlet settings in southeastern North Carolina, theflood tidal deltas also represent extensive sinks and chokepoints in the open . water and marsh infilled lagoons. Maintenancedredging of the Intra-Coastal Waterway at designatedaccess channels, is primarily due to the landwardtransport and deposition of reworked sediment from flooddeltas in the narrow lagoons.The great majority of the critical erosion zones that havebeen identified in Onslow Bay are associated with the 13contemporary inlets (Fig. 1 ) or those historic inlets whichwere closed artificially. From a geological standpoint inletsare far more important than their current physical dimensionsindicate. Less than one percent of North Carolina'sshoreline is occupied by inlets. Despite this low percentage,inlets during the past two centuries have influenced 65 % ofthe barrier shorelines that comprise the Onslow Bay compartmentand 100 % of some shoreline reaches. These percentagesare higher than those for other southeastern states.Inlet systems dictate the erosion and accretion patternsover long shoreline stretches, many times the current dimensionsof the typical inlet. The zone of influence is a functionof throat size, ebb shoal geometry and migration habit whendealing with locationally unstable inlets. In many casesdevelopment has encroached into these environmentally hazardouszones with disastrous results. From 1989-1995, 82%of the flood insurance claims for erosion threatened buildings(Upton/Jones) were along inlet influenced shorelines.These claims involved over nine million dollars in losses,almost 70 % of all erosion loss claims in North Carolina.During a previous three year period, 1978-1981, 60 of 70structures that were severely impacted by erosion were sitedalong shorelines directly influenced by inlets (Rogers per.Figure 1. Inlet locations. The 13 inlets long the Onslow Bayshoreline are a diverse group of stable and migrating systems.Eight inlets border developed shorelines, six of these have beenmodified to some extent. Stable inlets are common along theregressive barriers while locationally unstable systems are typicalof the transgressive barrier segment.com. 1995). Currently many other structures are threatened.Many North Carolina inlets have been modified bydredging for navigation purposes (Fig.1 ). These activitieswill likely continue as the coastal region continues to experiencerapid development and burgeoning economies. Beachnourishment is approaching the status of the only viablemeans of "preserving" developed beaches. Tidal deltas withnourishment quality sands will serve as future borrow sitesfor the rapidly eroding touristic beaches. Because of environmentalrestrictions regulating the activities within the narrowmarsh filled lagoons in southeastern North Carolina, the ebbdeltas are likely target areas. Large scale modification of severalebb deltas (Beaufort, Cape Fear River and Masonboro41

William J. ClearyPHOLOGYFigure 2. A. Bogue Banks. Forested beach ridges characterizeBogue Banks. Regressive barriers contain 15-25 times moresand per unit of length of coast than do transgressive barriers.B. Masonboro Island. Very low and narrow transgressive barriersare prone to overtopping during storm events. Inlet fill iscommon beneath the barrier and shoreface.Inlets) has resulted in dramatic changes along adjacentshorelines and significant morphological changes in the inletand its associated shoals (Cleary and Hosier 1987, 1989,1995; Cleary 1994).It is increasingly evident that the dynamics of inlets aresite specific, with each system exhibiting individualizedreponses to local environmental and geological factors. It isthe intent of this paper to provide a brief overview of thecoastwise variability of inlet types, morphological changeswithin the inlet system and the role the inlets play in the patternsof erosion and accretion on the adjacent shorelines. Theoverview is based on current investigations and publisheddata from studies of the majority of inlets in southeasternNorth Carolina.INLETS, EBB DELTAS AND SHORELINE MOR-Tidal inlets in southeastern North Carolina are mixedenergy (transitional) wave-influenced systems. At mixedenergy inlets (wave-dominated), a large portion of the varioussand bodies are concentrated within the inlet throat.Along southeastern North Carolina, natural inlets displaywell developed ebb deltas. Mixed energy or transitionalinlets are perhaps the most difficult to study due to the varietyof factors involved in determining the morphology of theinlet and associated sand bodies.Ebb-tidal deltas, the seaward shoals of an inlet areformed through the interaction of waves and tidal :currents.The general morphology of these features has been describedin detail beginning with the studies of Oertel (1972) andHayes, et al (1973). A number of studies have refined the initialmodels and described physical processes that shape thesefeatures (FitzGerald, 1976; Humphries, 1977; FitzGerald1984.) The overall morphology and the extent to which ebbdeltas are developed is a function of the inlet's tidal prismand the exposure to wave energy (Walton and Adams, 1976;Nummedal et al, 1977; FitzGerald, 1993; Hayes, 1994).Ebb tidal deltas along the Onslow Bay shoreline are reservoirsof good quality beach fill sand. The volume of sandcontained in these systems ranges from less than 750,000cubic meters to more than 80,000 million cubic meters.Slight changes in the size or shape of ebb deltas can have asignificant effect on adjacent shorelines (FitzGerald, et al,1978; FitzGerald and Hayes, 1980; Cleary and Hosier, 1987and 1989; Cleary 1994). Regardless of size, the offshoreshoals influence the ends of the barriers, acting as naturalbreakwaters and modifying the wave energy impinging uponthe shoreline. Waves approaching the islands are refracted insuch a manner that a region of sediment transport reversaloccurs downdrift of the inlet (Hayes, et al, 1973; Hayes,1980 and 1994).This mechanism of transport reversal had been proposedto account for the bulbous shoreline segment immediatelydowndrift of mesotidal inlets (Hayes et al, 1973). Cyclicalepisodes of complex bar- welding events account for a portionof the observed progradation (FitzGerald, 1984). Whenan inlet changes location or the symmetry (skewness) of theebb delta changes there is a concomitant change in :he patternof erosion/accretion on the adjacent shorelines (FitzGerald,.et al 1978; FitzGerald, 1984; Cleary and Hosier 1987and 1989; Cleary 1994).PREVIOUS STUDIES OF NORTH CAROLINAINLETSA number of North Carolina inlet studies exist. Theseinvestigations range in scope from the distribution and geologicsignificance of inlets (Cleary and Hosier, 1979, 1986a}to the US Army Corps of Engineers reports dealing with the42

INLET INDUCED SHORELINE CHANGES: CAPE LOOKOUT — CAPE FEARFigure 3. Brown’s Inlet. Brown’s Inlet, a stable system, islocated between Onslow Beach and Brown’s Island. Viewtoward the northeast and Cape Lookout.A series of barrier islands, spits and cuspate forelandsform the 185 km stretch of open ocean coast between CapeLookout and Cape Fear North Carolina (Fig.1 }. The OnslowBay coastal compartment is comprised of the low energyflank of the Cape Lookout Foreland and the northeasternhigh energy flank of the Cape Fear Foreland. This lowmesotidal shoreline segment is comprised of thirteen morphologicallydiverse barriers that can be grouped into twodistinct classes. The division occurs between Browns Islandand Onslow Beach where a submarine headland composedof Tertiary limestone and sandstone forms a prominent protuberancealong the coast. Onslow Beach perched atop theheadland separates the wide, high beach ridge and modifiedbeachridge barriers to the northeast from the transgressivebarriers with low narrow profiles to the southwest (Figs. 1 &2 A & 2 B).Overwash and historic inlet breaches are more commonalong the southern segment. The lagoons that back theregressive barriers, principally Bogue Banks, are shallow,open and generally free of tidal marsh. By contrast, marsheshave infilled the great majority of the southern lagoons. Thelack of tidal marsh behind Bogue Banks is directly attributableto the lack of significant inlet activity along the barrierduring its progradational phase during the past several thousandyears.Inlets along the regressive segment are generally locationallystable systems. This group includes those inletswhose throat section has remained in approximately thesame location (100 -200 m) for the past 75 years (Fig. 3).The minimum inlet width of this type varies from time totime and is related to throat expansion during storms andsubsequent constriction during storm free periods. Browns,Bear, Bogue and Beaufort Inlets (Fig. 1) are stable systemswhose locations are controlled by ancestral drainage patterns.Barden's Inlet, a small migrating inlet separatingShackleford Banks from Core Banks (Fig.1 ) is of relativelyrecent derivation having formed during a storm breach in the1920's.effects of dredging and navigation improvements such asthose for Masonboro Inlet (Vallianos, 1975; US Army Corpsof Engineers, 1982} and New River Inlet (US Army Corps ofEngineers, 1990}. Photographic reviews-currently out ofdate-depict, using sequential photographs or overlay maps,changes in inlet shorelines during a 35- 40 year period(Langfelder, et al, 1974; Baker, 1977}. An unpublished studyby Priddy and Carraway (1978} utilized a statistical analysisapproach to evaluate the recent migratory history of NorthCarolina inlets in an attempt to define an "inlet hazard zone".ONSLOW BAY SHORELINE OVERVIEWFigure 4. New Topsail Inlet. New Topsail Inlet located at thesouthern end of Topsail Beach is an unstable system that hasmigrated over nine km since the early eighteenth century.Small linear vegetated marsh islands paralleling the mainchannel record the inlet’s former positions.A second inlet type, typical of the transgressive barriersegment are locationally unstable systems that migrate atrates that range from 10- 100 m/y. Typically these systemsform during major storm events, generally seaward of smallincised coastal plain estuaries and migrate in a down driftdirection (southwest). Several of the current inlets, such asNew Topsail (Fig. 4), have been in existence for several centuries.Many others that formed subsequent to New Topsailin the eighteenth and nineteenth centuries are now closed.The position of the former inlet is now marked by extensivetidal marsh that has colonized the relict flood deltas.Several inlets in the transgressive segment, includingOld Topsail and Mason's, are in various stages of closure.New River, New Topsail, Old Topsail, Mason's, Masonboro(now stabilized) and New Inlet are migrating systems (Fig.5A-D). Rich Inlet (Fig. 5 E) is the only naturally stable systemin the southern barrier shoreline reach. Carolina Beach43

William J. ClearyFigure 5. Inlets along the transgressive barriers.A. New River Inlet. A slowly migrating system located between Onslow Beach and Topsail Island.B. Old Topsail Inlet. A migrating system near closure.C. Mason’s Inlet. A small migrating system located between Shell Island and Figure Eight is approaching the closure phase.D. Masonboro Inlet. A dual jettied inlet located between Wrightsville Beach and Masonboro Island.E. Rich Inlet. Located at the north end of Figure Eight Island, Rich’s inlet is the only natural stable system along the transgressivebarrier shoreline segment.F. Carolina Beach Inlet. Located at the northern end of Carolina Beach, the artificially stable inlet was opened in 1952. Periodicdredging allows the inlet to remain open.44

INLET INDUCED SHORELINE CHANGES: CAPE LOOKOUT — CAPE FEARInlet (Fig. 5 F) was artificially opened in 1952 and its stabilityis artificially maintained through dredging.METHODOLOGYDocumentation of shoreline positions and former inletlocations were based in part upon data derived from historicmaps and charts that date from 1857. A number of additionalcharts are also available from the early 1700'5. Historicalaerial photographs dating from 1938-1995 were used todetermine changes in shoreline positions adjacent to an inletand morphologic changes in the inlet throat and its associatedebb delta. The number of sequential aerial photographsavailable for each inlet varied from 20-75 sets. The photographswere analyzed for temporal and spatial changes ininlet and ebb delta morphology. Photographs taken atmonthly or quarterly intervals were available from theU.S.A.C.E. Wilmington District Office, for portions of thelast 20 years for some inlets. Photographs were digitized andbaselines were established and relative to these a number ofparameters were measured.Bathymetric charts were used to the greatest extent possibleto determine the characteristics of each inlet system.Limited bathymetric charts and partial surveys of the throatand seaward extension of the channels were consulted to provideanother dimension. Valuable hydraulic data were determinedfrom the bathymetric charts and used to compare theevolution of the system during pre- and post dredging periods.The variables measured included cross-sectional area(Ac), inlet minimum width (W), Depth (D) and ebb deltavolume among others. Tidal prism and ebb delta volume datawere calculated in some instances utilizing the methodologyof Jarrett (1976) and Walton and Adams (1976).Figure 7. Relationship of inlet stability, local drift directionand the direction of movement of bypassed bar packets followingebb delta breaching events. Size of packet bypassed is afunction of shoal size and cycle duration.INLET MORPHODYNAMICSCurrent and recently published studies of the inletsalong the Onslow Bay shoreline have shown that the systemsthat comprise the two major categories of inlets are quitediverse. Analyses of the published and unpublished data setsindicate the following generalizations apply to inlets in thetwo major barrier reaches that comprise the Onslow Bayshoreline.1 .Erosion and accretion cycles along stable inlet shorelineswere related to cyclical changes in the symmetry of ebb deltas(Fig. 6). The cycles were associated with repositioningand reorientation of the main ebb channel and the correspondingflood channels and where swash bars attached tothe adjacent barriers. Cycles ranged in duration from five to25 years and cycle length correlated with inlet size and stormhistory. Cycles were shortened by storms: cycles were typicallylonger in larger inlets.Figure 6. Erosion along stable inlet shorelines. Cycles of ebbchannel deflection, and consequent repositioning of marginalflood channels prompts erosion on downdrift shoulder. Repositioningthrough deflection or reorientation of ebb channel duringstorms promotes accretion. Cycle is of variable duration.Figure 8. Rich Inlet. Oblique aerial photograph of Rich Inlet(March 1995). Southward deflection of the ebb channeltoward bottom of photo initiates a cycle of erosion on FigureEight Island. Deflection prompts repositioning of flood channel(F) and subsequent erosion. Reorientation of ebb channelto the north (toward top of photo) initiates accretion on FigureEight shoulder as bars migrate through flood channel (openarrow) and attach to barrier. Photograph illustrates channelin an early phase of renewed deflection.45

William J. ClearyFigure 9. Shoreline and ebb delta changes at Bald Head Island,1857-1985. A mid 19 th century deflection of the entrance channel(arrow) promoted large scale erosion of the Bald HeadIsland shoreline. A shore normal repositioning of the ebbchannel in the 1880’s triggered the bypassing of 8-10 millioncubic m of sand to the Bald Head Island shoreline. A fixedchannel position and the segmentation of the ebb delta hasdrastically reduced the downdrift supply and prohibited barbypassing.The process of channel extension and abandonment providesa mechanism at some inlets whereby sand packets ofvarying size (range 5×10 3 to 4.0×10 6 ) are generallybypassed downdrift and ultimately to the adjacent barriershoreline (Fig. 7).The northern portion of Figure Eight Island adjacent toRich's Inlet. a stable system that forms the northern boundaryof the island. accreted more than 200 m from 1985-1993following a reorientation of the ebb channel and flood channelrepositioning (Fig. 8). In adjacent Long Bay, Bald HeadIsland. the barrier adjacent to the Cape Fear River Inletaccreted southwestward more than 800 m from 1885 to 1915following a breaching event (Fig .9). This large scale accretionepisode pre-dated the major modifications of the harborentrance channel.Figure 10. Trailing barrier realignment. Updrift erosion is aconsequence of inlet migration and the associated planformadjustment.2. Locationally unstable inlets are generally restricted to thetransgressive barrier segment. These systems migrated to thesouthwest at average annual rates varying from 5.0 to 150 m/yr. In addition to the erosion of the downdrift barrier, migra-Figure 11. Aerial Photographs of Topsail and Figure EightIslands.A. Topsail Beach. Topsail Beach updrift of New TopsailInlet receded as much as 160m in some areas (1970-1985)due to realignment of the trailing shoreline. South view.B. Figure Eight Island. Chronic erosion of the southern 4km of the barrier stems from the platform adjustment of theupdrift shoreline as migration occurs. North view.46

INLET INDUCED SHORELINE CHANGES: CAPE LOOKOUT — CAPE FEARFigure 12. New Topsail Inlet. Ebb delta breaching and bar updrift bypassing at New Topsail Inlet. Portions of several cycles areillustrated. Note ebb delta symmetry changes and ebb channel (ec) orientations. Also note repositioning and expansion of the floodchannels (fc).tion resulted in truncation and realignment of the trailingshoreline (Fig. 10).Rates of updrift shoreline recession ranged up to 12 mi yfor as long as a decade. Erosion rates decreased as the updriftbarrier planform adjusted to the position of the inlet. Thechronic erosion zones along the southern portions of TopsailBeach and Figure Eight Island are located updrift of NewTopsail Inlet and Mason's Inlet respectively (Figs. 11 A &B). Erosion stems from the migration of the adjacent inlets.The success of beach restoration projects along these shorelinesegments is severely limited due the planform changesdictated by the locationally unstable inlets.Concurrent with the processes of migration, packets ofsand were bypassed to the updrift shoulder of the inlets whenebb deltas were breached. The cycle of ebb delta breachingvaried in length from 2-20 years (Fig. 12 A-F). Reorientationof the updrift channel and simultaneous changes in the symmetryof the ebb deltas caused migration and attachment of47

William J. Clearymajor reduction in sand supply, and wash over topographyhas increased along 80 % of the island's length.INLETS AND MANAGEMENT PERSPECTIVESFigure 13. New River Inlet. Line drawing depicting the 1938and 1986 shoreline positions adjacent to the inlet. OnslowBeach (right) has recessed at rates as high as 8 m/y since themid 1960’s when major dredging efforts began. Although conjectural,it appears dredging has promoted an increase in thetidal prism (40%) and the retention capacity of the ebb delta.Note the accretion on the downdrift shoulder. Channel positionand orientation controls accretion trends.small bar complexes (100x50m) within 1 km updrift of theinlet's ebb channel. Frequently these temporary shorelineconvolutions promoted rapid erosion leeward of the locationwhere the sand packets attached. The attachment of sandpackets frequently prompts periods of relatively rapid inletmigration as bars move laterally along the barrier's spit complexand into the adjacent bar built estuary.3. Modifications of inlets by dredging resulted in an increasein the tidal prism, a corresponding larger retention capacityof the ebb delta (Fig. 13) and the disruption of the naturalebb delta breaching cycle. Dredging changes are reflected inan extension and deepening of the ebb channel across theebb platform. Bypassing to the downdrift shoal segment andthe adjacent shoreline ceases when the deepened ebb channelbifurcates the the ebb platform forming distinct shoal segments.Bypassing generally decreases as channel maintenanceincreases. Shoreline erosion occurs when sand supplyis reduced by major inlet dredging. Typically the maximumrate of shoreline erosion and ebb delta morphological changemay lag behind the breaching of the shoals by severaldecades, especially at larger inlets such as Beaufort Inlet(Fig. 14) or at Cape Fear River Inlet (Fig. 9) in adjacentLong Bay.Inlet stabilization by jetties mimicked the impacts ofdredging. The modification of inlets by hard structures hasreduced sand bypassing and increased erosion on one or bothsides of the inlet. Significant long term coastwise erosionassociated with inlet modification is exemplified by MasonboroIsland a 13 km long transgressive barrier. The barrier islocated between a dual jettied system at Masonboro Inlet anda continually dredged artificial system at Carolina BeachInlet (Figs. 1 & 5F). Masonboro Island has experienced aThe decade to century scale coastwise sand budget andultimately the shoreline retreat rate are negatively affected bythe increased retention capacity of modified inlets. Becauseof the large number of inlets and the storm frequency insoutheastern North Carolina, sand loss to the adjacent shorefaceis likely to be much greater than in regions where naturalsystems occur.The designation of inlets as areas of environmental concernnecessitates special consideration when dealing withthese systems. The dynamic nature of inlets coupled with ourinability to predict the magnitude and direction of change,makes management decisions very difficult. Effective managementrequires an understanding of the processes both naturaland man-induced that produce changes. Each inlet isunique and site specific management strategies must bedeveloped for planning purposes.North Carolina has been a pioneer in developing oceanfrontmanagement tools. Although the state's inlet hazardstandards are in need of review, it is one of a very few statesthat attempts to manage shorelines near unstabilized inlets. Itis likely that North Carolina will achieve this goal due to theincreased awareness of the hazards associated with inlets.REFERENCESCleary, W.J., 1994, New Topsail Inlet, North Carolina. Migrationand Barrier Realignment: Consequences for Beach Restorationand Erosion Control Projects. Union Geographique Internationale,Commission Sur de l'Environment Cotier C, Institute deGeographique p. 116-130.Cleary, W.J., and Hosier, P.E., 1987, Onslow Beach, N.C.: Morphologyand Stratigraphy, Proc., Coastal Sediments '87, NewOrleans, p. 1760-1775.Cleary, W. J. and Hosier, P.E., 1995, Morphology and Stratigraphyof a Transgressive Barrier and Associated Estuarine System,Masonboro Island, NC, Final Reprt, National Estuarine SanctuaryResearch Reserve Program, 25 p.Cleary, W.J., and Hosier, P.E., and Gammill, S., 1989, Natural andDredging Related Shoreline Changes, Bald Head Island, CapeFear, N.C., in CZM '89, Amer. Soc. Civil. Eng., no.19, v. 4, p.2017-3029.FitzGerald, D.M. 1976. Ebb- Tidal Delta of Price Inlet, South Carolina:Geomorphology, Physical Processes and Associated InletShoreline Changes in Hayes, M.O.; Kana, T.W. eds. TerrigenousClastic Depositional Environments. Tech., Rept. No. II-CRD, Coastal Research Division, Dept. Geol., University ofSouth Carolina; p.II-158-171.FitzGerald, D.M. 1984. Interactions Between the Ebb- Tidal Deltaand Landward Shoreline: Price Inlet, South Carolina, J. Sed.Pet. 54: 1303-1318.FitzGerald, D.M., Hubbard, D.K., and Nummedal, D. 1978. Shore-48

INLET INDUCED SHORELINE CHANGES: CAPE LOOKOUT — CAPE FEARFigure 14. Beaufort Inlet and ebb delta changes. Shoreline and ebb delta morphological changes are directly related to the directionand repositioning of the ebb channel. Dredging of the ship channel since the 1930’s has resulted in a deepening and seaward growthof the ebb delta platform. The seaward extension of the shoals has steepened the nearshore profile promoting erosion in some areas.Approximately 20 million cubic meters of sediment has been lost from the ebb delta since 1936 due to a combination of factors.These include annual dredging, storm losses to the shoreface and the shoreward transport sand packages responsible for the westwardgrowth of the Shackelford Bank’s spit (Modified after USACE 1976).This represents UNCW’s Center for Marine Science Research contribution #149.49

William J. Clearyline Changes Associates with Tidal Inlets along the South Carolina:Coast. Coastal Zone '78, Symposium on Technical,Environmental, Socio-economic and Regulatory Aspects ofC.Z.M., ASCE; San Francisco.Hayes, M.O., Owens, E.H., Hubbard, D.K., and Abele, R.W. 1973.The Investigation of Forms and Processes in the Coastal Zone,in Coates, D.R., ed. Coastal Geomorphology Publications inGeomorphology, Birmingham, NY, State University of NewYork, p. 11-41.Hayes, M.O. 1980. General Morphology and Sediment Patterns inTidal Inlets. Sed. Geol. 26:139-156.Hayes, M.O., 1994, The Georgia Bight barrier system: in Davis(ed), Geology of Holocene Barrier Islands systems., SpringerVerlag, Chapter 7, p. 233-305.Humphries, S.M. 1977. Morphologic Equilibrium of a NaturalTidal Inlet: American Society Civil Eng., Proceedings CoastalSediments '77, Charleston, SC. p. 986-1005.Jarrett, J. T., 1976, Tidal Prism- Inlet Area Relationships: GeneralInvestigation of Tidla Inlets (GITI), Report 3, U.S.Army Corpsof Engineers, 76p.Langfelder, L. J., French, T., McDonald, R., and Ledbetter, R.,1974. A Historical Review of Some of North Carolina's CoastalInlets. Center for Marine and Coastal Studies, NCSU. Raleigh,NC. Report No.74-1 , 43p.Nummedal, D.N., Oertel, G.F., Hubbard, D.K., and Hine, A.C.1977. Tidal Inlet Variability - Cape Hatteras to Canaveral.Coastal Sediments, '77 , Proceedings, Fifth Symposium of theWaterway, Port, Coastal and Ocean Division of the ASCE,Charleston. SC. p. 543-562.Oertel, G.F. 1972. Sediment Transport of Estuary Entrance Shoalsand the Formation of Swash Platforms: J. Sed. Pet. 42:857863.Priddy, L.J. and Carraway, R. 1978. Inlet hazard areas. The FinalReport and Recommendations to the Coastal Resources Commission.North Carolina Dept. of Nat. Resources and CommunityDevelopment, Raleigh, NC. 51 p.U.S. Army Corps of Engineers, 1976. Morehead City Harbor, NorthCarolina, General design Memorandum, U.S. Army Corps ofEngineers, Wilmington District, Wilmington, NC.U.S. Army Corps of Engineers, 1990, Final Feasibility Report andEnvironmental Impact Statement on Hurricane Protection andBeach Erosion Control, West Onslow Beach and New RiverInlet, North Carolina, Wilmington District, Wilmington, N.C.,74 p. and appendices.Walton, T.L., Jr. and Adams, W.D. 1976. Capacity of inlet outerbars to store sand. in Proc. Conf. Coastal Engineering, NewYork: ASCE, Vol. 2, p. 1919-1937.50

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 51 - 57SEDIMENTOLOGY AND DEPOSITIONAL PROCESSES IN THE TIDAL MARSHES OF SOUTHEASTERNNORTH CAROLINALynn A. LeonardDepartment of Earth SciencesUniversity of North Carolina at WilmingtonWilmington, NC 28403-3297ABSTRACTBroadly speaking, two different types of coastal marshexist in southeastern North Carolina; back barrier marshesand tidal creek marshes. These marsh systems differ onefrom another in terms of surficial sediment characteristicsand in terms of the physical processes that control sedimentdeposition. Back barrier marshes consist of predominantlysand sized, inorganic, quartzose sediment deposited underthe influence of episodic, high energy processes such aswave induced washover. In contrast, tidal creek marsh sedimentsare fine grained, organic deposits which accumulateunder steadier, low energy conditions. The sedimentology ofboth systems has been impacted by human's alteration of thenatural environment.INTRODUCTIONThe southeastern North Carolina coastline is markedlydifferent from other segments of the coast. Unlike the "OuterBanks" region to the north, which possesses barrier islandsseparated from the mainland by wide back barrier lagoons,the coastline from Cape Lookout to Cape Fear is characterizedby densely vegetated, narrow lagoons (average width of1.5 km) which lack major fluvial inputs and which may bedissected by winding tidal channels extending from the shorttidal creeks which drain adjacent uplands (Cleary et al.,1979). Within this coastal setting, two different types ofcoastal salt marsh are present (Figure 1); back barriermarshes (e.g. Masonboro Sound) and marshes associatedwith incised, mainland, tidal creek systems (e.g. Hewlettsand Bradley Creeks).It is currently believed that most southeastern US marshsystems formed over the last 5-6000 years as shallow marinebasins were infilled during a slow rise of sea-level. The ultimatedevelopment of individual systems, however, isstrongly controlled by the inherent variability of the prevailingphysical processes. Back barrier marshes, for example,are strongly impacted by high energy processes due to theirproximity to the open ocean. The development of flood tidaldeltas during inlet migration and the formation of extensiveoverwash deposits during periods of increased wave activity(Cleary et al., 1979) are two mechanisms which contributethe large volumes of sedimentary material required by thesesystems to "keep pace" with local sea level rise. The largeFigure 1. Locations of marsh areas referred to in text.Transect lines, boxes and numerals refer to locations referencedin subsequent figures.sand influxes associated with episodic, high energy events(Hackney and Cleary, 1987) and with inlet migration, however,are usually localized. As a result, the ability of localback barrier marshes to maintain their position with respectto sea level is highly variable.Deposition within protected, mainland tidal creekmarshes also results from increased inundation associatedwith a gradual rise in sea level. According to radiocarbondates obtained from cores collected in :one local tidal creek(Hewlett's Creek), the leading edge of the Holocene transgressionbegan onlapping the surface of the subaerial Pleistoceneand the alluvial valley fill of ancestral Hewlett'sCreek by 5300 BP (Figure 2). By about 4200BP, floodinghad reached the present upper estuary and conditions becameoptimal for salt marsh development (Berger, 1993). data suggestthat salt marshes have continuously occupied local tidalcreek basins since 4000 BP. In contrast to back barrier marsh51

Lynn A. LeonardFigure 2. Stratigraphic transect of Hewletts Creek. Transect locations shown in Figure 1. (Modified from Berger, 1993).systems, deposition in tidal creek marshes has been controlledmostly by low energy, tidal processes since theirestablishment. The aim of this paper is to describe the sedimentsand the modern physical processes controlling sedimentationin both back barrier and tidal creek marshenvironments.MODERN SEDIMENTSTidal CreeksThe surficial sediments of local tidal creek basins consistpredominantly of very fine to fine sands, althoughmedium to coarse sands and gravels may occur in tidal creekchannels (Figure 3).Dry sieve analyses of surface samples collected withinone and one-half kilometers of the mouth of Hewlett's Creek(Berger, 1993) indicate that from 77 to 84 weight percent ofsurficial sediments exhibit diameters in the range of 88 mmto 125 mm (fine to very fine sand). These sediments typifythose found on tidal channel floors and on extensive subtidaland intertidal flats. In the landward reaches (>1.5 km frommouth) of the tidal creek basins, where tidal flats give way tovegetated marshes, the relative abundance of sands decreaseswhile silts and clays become more abundant. A characteristicmarsh deposit generally contains less than 25 weight percentsand, 30 percent silts and as much as 50 percent clays. Inaddition, sediments in the landward reaches of tidal creekbasins exhibit higher organic contents than their lower basincounterparts. The organic content of surficial marsh sedimentsaverage 18-22% (Leonard, 1995), while the organiccontent observed for tidal flat and tidal channel sedimentsmay be less than one percent (Steenhuis, 1994).Back BarrierThe characteristics of back barrier marsh sediments differfrom the marsh sediments found in tidal creek basins. Ingeneral, back barrier marsh sediments are coarser and moreclosely resemble sediments found near tidal creek mouths.Grain size analyses conducted in marshes behind Topsail(Gamill, 1990) , Masonboro (Steenhuis, 1994), Figure Eightand Shell Islands (Metz and Leonard, 1996) indicate thatback barrier marsh sediments generally contain more than70% sand. Within the sand fraction, the majority of the materialusually consists of medium to fine sands (0.5mm-0.25mm). As much as 27%, however, may fall within thecoarse (>0.5 mm) sand class. Fine grained sediments (i.e.52

SEDIMENTOLOGY AND DEPOSITIONAL PROCESSES IN THE TIDAL MARSHES OF SOUTHEASTERN NORTH CAROLINAdecreases with proximity to major drainage channels and toinlets (Metz and Leonard, 1996) (Figure 4). Sediments collectedin different marsh environments behind MasonboroIsland (Steenhuis, 1994), contained less than 10% organiccarbon by weight in the top 10 cm. The mean organic carboncontents of high and low marsh sediments are 5.1% and5.3%, respectively. Very low organic carbon contents occurin intertidal flat (1.3%) and channel (0.8%) sediments. Thelow organic content of back barrier marsh sediments, asopposed to the tidal creek systems, has been attributed to theepisodic influx of marine sands as overwash fans duringstorm activity (Hosier and Cleary, 1977) and to the reworkingof coarse sediments derived from local dredge spoildeposits (Steenhuis, 1994).MODERN PROCESSESFigure 3. Facies map of surficial sediments in Hewletts Creekbasin (modified from Berger, 1993).silts and clays) comprise less than 20% of atypical depositalthough high percentages of clay may be found in isolatedsalt pond environments (Steenhuis, 1995).Organic contents, estimated from weight loss on ignitionat 400°C, are highly variable and range from 25% to 8%for back barrier marsh sediments. In general, organic contentFigure 4. Organic content of marsh sediments along a N-Stransect in the vicinity of Mason’s Inlet. Site locations areshown in Figure 1.The sedimentologic signature of each marsh area isstrongly influenced by the types of physical processes activein each area. While sediment transport and deposition inlower energy tidal creek marshes is dominated by fairly predictableand steady processes such as spring/neap tidal variabilityand biologic activity, episodic and often catastrophiccoastal processes dominate sedimentation in the back barriermarshes of southeastern NC. In these systems, whose formationis closely linked to the deposition of washover fans andflood tidal deltas (Hosier and Cleary, 1977), tidal inlet processesplay an important role in maintaining marsh surfaceelevation.A number of different techniques (e.g. marker horizons,sediment traps, aerial photograph surveys) have beenemployed to assess the extent to which modern inlets influencesediment supply in back barrier marshes. The results ofthese studies (e.g. Hackney and Cleary, 1987; Gammill andHosier, 1992; Metz and Leonard, 1996) have concluded thatinlet processes both constructively and destructively impactback barrier marsh sedimentation. The migration of inletchannels actively erodes adjacent marshes while simultaneouslydepositing large volumes of sand (Plate 1 A) whicheffectively bury the vegetated marsh surface. Concurrently,the movement of sands associated with the inlet's flood tidaldelta infill existing channels and provide substrate for newmarsh development (Plate 1 Band C). In addition, sandsimported through the inlet throat provide inorganic materialcritical to the overall accretionary budget of these systems.Sediment trap data have shown (Figure 5) that maximumdeposition occurs in close proximity to the inlet and that depositionrates decrease with distance from the inlet. Anecdotal(Metz pers. comm.) and empirical (Gammill, 1990) evidencesuggest that marshes removed from the influence of the tidalinlet are soupy, lack well developed rhizomatous mats, showan increased number of ponds and overall exhibit characteristicsindicative of marsh deterioration. These observationsare consistent with the findings of Hackney and Cleary53

Lynn A. LeonardFigure 5. Deposition rates measured by sediment traps along aN-S transect in the vicinity of Mason’s Inlet. Site locations areshown in Figure 1.Plate 1A. Remnants of marsh buried by overwash deposits.B. Infilling of Mason’s Inlet by sands.C. Establishment of vegetation on newly deposited inletsands.(1987) who suggested that the lagoonal marshes of southeasternNorth Carolina would eventually be drowned by risingsea level without the input of marine sands throughinlets.Storm sedimentation is also an essential component ofthe vertical accretionary budget of back barrier marshes.Cleary et al. (1979) conducted a regional investigation ofmarsh islands present within local bar- built estuaries andconcluded that their formation resulted from storm waveactivity and increased wave swash which transported sandfrom the flood tidal deltas onto the adjacent marsh. Recentdata collected in the vicinity of Mason's Inlet concur withthese observations. Maximum deposition in the back barriermarsh behind Wrightsville Beach occurred during the offshorepassage of Hurricane Gordon in November 1994 (Figure6). Deposition resulting from the elevated tides andincreased wave activity associated with this event wereroughly 6 to 10 times deposition rates measured during nonstormconditions.Sediment transport in local tidal creek systems is notdominated by the occurrence of episodic, high energy events.Instead, deposition in these protected marshes is controlledby the diurnal inequality of the tides and spring/neap variability.Sediment transport is also strongly influenced by seasonalvariability and the occurrence of meteorologic events(e.g. low tide rainstorms, tropical storms and 'nor'easters').Flux studies conducted within two local tidal creek systems(Hewletts and Bradley Creeks) indicate that maximum sedimenttransport occurs during spring tides when stronger tidalcurrents and higher water levels occur. Maximum sedimenttransport is also favored during summer months when waterlevels are typically higher and when increased levels of bioturbationcontribute to particle disaggregation (i.e. increasemobility).Sediments in suspension consist primarily of very fine54

SEDIMENTOLOGY AND DEPOSITIONAL PROCESSES IN THE TIDAL MARSHES OF SOUTHEASTERN NORTH CAROLINAFigure 6. Deposition measured by sediment traps deployedduring storm and non-storm conditions in the marshes behindWrightsville Beach, NC. Site locations are shown in Figure 1.sands, silts, clays and organic aggregates. The coarse gravelsobserved on channel floors are not usually resuspended bythe 40 cm S-1 or lower flow velocities within the creek during'non-meteorologically' forced conditions. Instead thesematerials are most likely transported as bedload.Suspended sediment flux analyses within Bradley Creek(Angelidaki and Leonard, 1996) suggest that fine grainedsediments and organic aggregates are imported into theupper reaches of local tidal creek estuaries under normaltidal conditions. This process may be expedited followingFigure 7. Deposition measured by sediment traps deployed inthe Bradley Creek marsh basin. Traps were deployed withinthe boxed area shown in Figure 1.Figure 8. Marsh elevation and sediment deposition measuredalong a NW-SE trending transect in the Bradley Creek marshbasin. Location of the transect is shown by inset. Location ofthe inset is shown in Figure 1.periods of intense rain when runoff from adjacent uplandsmay increase total suspended solid concentrations to levels10X those occurring during fair weather. These flux measurementscorroborate Berger's (1993) observation that sedimenttransport processes are resulting in the deposition ofmuds in the upper reaches of tidal creek basins and alsoreflect the observations of long time residents who reportthat tidal creeks have infilled over the last 50 years.Once transported into the estuary, sediments are depositedon the marsh surface during inundation. Sediment trapdata indicate that deposition on the marsh surface is highlyvariable ranging from a minimum of 13.8.:1:. 3.4 g m-2 to63.7.:1:. 10.3 g m-2 (Leonard, in review). For the most part,sediment deposition mimics the seasonal variabilityobserved in suspended sediment data. Deposition rates measuredduring summer months exceed deposition measuredduring winter months (Figure 7).These results are consistent with the seasonal depositionaltrends reported for Gulf of Mexico marshes byLeonard et al. (1995) and for Georgia marshes by Letzschand Frey (1980). Variability observed in the winter data maybe attributed to storm activity (Reed 1989, Childers and Day55

Lynn A. LeonardFigure 9. Surficial flow patterns observed during a spring tidalinundation at Bradley Creek. A) Initial flooding of marsh surface,B) sheet flow during mid to late flood, C) initiation of ebbfollowing slack high water, and D) late ebb.1990) and extreme tides. Data collected in November 1994coincided with the offshore passage of Hurricane Gordonand the December 1994 data were collected during a perigeanspring tide.Sediment accumulation is also influenced by proximityto potential sediment source and the prevailing hydrodynamicconditions on the marsh surface. Figure 8 shows sedimentdeposition (as measured by surficial sediment traps)plotted along an E-W trending marsh transect and illustratesthree important depositional trends. First, the data indicatethat deposition both increases and becomes more variable inthe seaward (downdip) direction. Second, the data suggestthat deposition on creek margins exceeds deposition in themarsh interior (> 3m from the creek). Lastly, Figure 8 showsthat sediment deposition on the landward bank of a creekexceeds deposition on the seaward bank. These depositionalpatterns can be related to variations in source proximity.Ultimately, there are two potential pathways by whichallocthonous sediments are transported into a tidal creekmarsh. The first occurs when water levels exceed bank fullconditions and sediments are transported out of the tidalcreek and onto the marsh. The second occurs when sheetflows (for this scenario, a sheet of water moving in onedirection across the marsh surface) are established. Tidalcreek spillage (pathway number one) may be invoked toexplain increased deposition on creek margins relative tomarsh interior sites, while sheet flow may account forincreased deposition in the seaward direction.In order to explain the observed differences in depositionon landward creek margins relative to seaward creekmargins, both surficial flooding patterns and creek hydrologymust be addressed. For example, peak flood currentvelocities in Bradley Creek occur at two separate times; thefirst coinciding with bank full conditions and the secondwith sheet flow conditions. During bank full conditions, flowmovement is primarily creek normal (Figure 9) such that thepotential for sediment transport out of the creek is equallyfavorable for both sides of the creek. During the secondvelocity pulse, the one occurring during sheet flow, sedimentwill be transported onto the marsh surface along landwardcreek margins only. At seaward creek margins, the dominantdirection of transport is off of the marsh surface even thoughwater levels are still rising.For systems dominated by sinuous creeks, such as theBradley and Hewletts Creek systems, the implication on sedimentdeposition is two-fold. First, maximum depositionshould occur on landward margins where sediment ladenwater is first reaching the marsh surface from the energeticcreek channel. Second, sediment deposition should be lesson seaward margins since the flood water reaching there duringsheet flow will have already traversed a vegetated systemand presumably lost much of its suspended material in themarsh interior. This hypothesis is corroborated by depositiondata shown in Figure 8 which indicate that deposition on thelandward margins of tidal creeks exceeds deposition on seawardmargins.Increased deposition during the summer is an observationconsistent with the findings of earlier studies (Letzschand Frey 1980, Leonard et al. 1995) . Such increases, however,are usually attributed to increased bioactivity such asburrowing, algal production and fecal pellet production(Harrison and Bloom 1977, Leonard et al. 1995), increasedwater levels in the summer (Ward 1981) or seasonal changesin water viscosity (Leonard et al. 1995). Another possibleexplanation is a physical mechanism; specifically, that thebaffling of over marsh flows is enhanced in the summerwhen the availability of live plant material on the marsh surfaceis at a maximum (Leonard, in review).SUMMARYThe tidal marshes existing within local tidal creek systemsand behind back barrier marshes can be differentiatedone from another in terms of both their sedimentologic signatureand the depositional processes. Despite their differences,however, both systems have been impacted by man'sactivity to the extent that the natural evolution of each areahas been altered.Historically, estuarine marsh systems in southeasternNC have been most significantly impacted by the construc-56

SEDIMENTOLOGY AND DEPOSITIONAL PROCESSES IN THE TIDAL MARSHES OF SOUTHEASTERN NORTH CAROLINAtion of the Atlantic Intracoastal Waterway (AIWW). Dredgingof the waterway in 1932 and the construction of dredgespoil islands have altered the existing sedimentary andhydrodynamic connection between the mouth of tidal creekestuaries and the back-barrier environments. A comparisonof maps from 1857 and 1933 (Berger, 1993) has shown thatnear the mouth of Hewletts Creek, the number of smallmarsh creeks has decreased and that the marsh area hasexpanded by approximately 10-15% adjacent to the AIWW.Aerial photograph analyses (Berger, 1993) , show similarincreases in marsh area between 1949 and 1990 and anincrease in shoaling within tidal creeks (i.e. formation ofintertidal or subtidal sand flats, and oyster flats). These analysesfurther suggest that the modifications are most severe inthe lower portions of the estuaries as opposed to the upperestuaries. Anecdotal and aerial photographic evidence suggestthat much of the morphological change within theseestuaries has occurred over the past 10 to 20 years.Data presented in this paper have demonstrated the criticalrelationship between inlet processes and the ability of aback barrier marsh to maintain its elevation with respect tosea level rise. While many marsh sites are presently accretingat rates (1.5 mm y-1 or more) sufficient to keep pace withlocal rates of sea level rise, others are not (Gammill andHosier, 1993). Given the essentially ephemeral nature ofthese systems, it is clear that they will be highly susceptibleto changes in sediment supply.Hackney and Cleary (1987) report that human activitiesalong the coast, such as the construction of jetties and dredgingof inlets, are disrupting the transport of sand into backbarrier lagoons. While Hackney and Cleary (1987) reportthat it is premature to adequately predict whether currentdredging frequency is having a significant impact on sandtransport to marshes, the available data suggest that the probabilityof marsh submergence will be hastened by continuedsand removal coupled with increases in the rate of sea levelrise.REFERENCES .Angelidaki, K. and L.A. Leonard. 1996. Sediment transport patternsin a small tidal creek; Bradley Creek, NC. In: GeologicSociety of America Southeastern Section Annual Meeting, Programswith Abstracts. Jackson, Mississippi, p. 11.Berger, Julia H. K. 1993. Response to Sea Level Rise in a smallCoastal Plain Estuary, Southeastern North Carolina. UnpublishedMaster's Thesis, University of North Carolina at Wilmington,Wilmington, NC. 63p.Childers, D.L and Day, J.W., Jr. 1990. Marsh-water column interactionsin two Louisiana estuaries. 1. Sediment dynamics. Estuaries,13: 393-403.Cleary, William J., Paul E. Hosier, and Glenn R. Wells. 1979. Genesisand significance of marsh islands within southeastern NorthCarolina lagoons. J. Sed. Pet. 49(3): 703-710.Gammill, Steven P. and Paul E. Hosier. 1992. Coastal saltmarshdevelopment at southern Topsail Sound, North Carolina. Estuaries15(2): 122- 129.Gammill, Steven P. Coastal Saltmarsh Development at SouthernTopsail Sound, North Carolina. Unpublished Master's Thesis,University of North Carolina at Wilmington, Wilmington, NC.51 p.Harrison, E.Z. and A.L. Bloom. 1977. Sedimentation rates on tidalsalt marshes in Connecticut. J. Sed. Pet., 47: 1484-1490.Hackney,Courtney T., and William J. Cleary. 1987. Saltmarsh loss inSoutheastern North Carolina lagoons: Importance of sea levelrise and inlet dredging. J. Coast. Res. 3(1 ): 93-97.Hosier, Paul E. and William J. Cleary. 1977. Cyclic geomorphicpatterns of washover on a barrier island in southeastern NorthCarolina. Env. Geol. 2: 23-31.Leonard, L.A. 1995. Sediment deposition in a North Carolina backbarrier marsh system: patterns, processes and storms. ill: SoutheasternRegional Geological Society of America Annual Meeting1995 Abstracts with Programs, Knoxville, Tennessee, p. 69.Leonard, L.A., A.C. Hine, and M.E. Luther. 1995. Surficial sedimenttransport and depositionprocesses in a Juncus roemerianusmarsh, West-Central Florida. J. Coast. Res., 11 (2): 322-336.Leonard, L.A. 1996. Controls of Sediment Transport and Depositionin an Incised Mainland Marsh Basin, Southeastern NorthCarolina. Wetlands (in review).Letzsch, S.W. and R.W. Frey. 1980. Deposition and erosion in aHolocene salt marsh, Sapelo Island, Georgia. J. Sed. Pet. 50:529-542.Metz, K. and L. Leonard. 1996. Inlet migration and sedimentationon back barrier tidal marshes. ill: Geologic Society of AmericaSoutheastern Section Annual Meeting, Progarams withAbstracts. Jackson, Mississippi, p. 12.Reed, D.J. 1989. Patterns of sediment deposition in subsidingcoastal salt marshes; the role of winter storms. Estuaries 12,222-227.Steenhuis, Joanne, H. 1994. Holocene Sedimentology of a MasonboroIsland Estuary, Masonboro Island, North Carolina.Unpublished Master's Thesis, University of North Carolina atWilmington, Wilmington, NC. 73p.Ward, L.G. 1981. Suspended- material transport in marsh tidalchannels, Kiawah Island, South Carolina. Mar. Geol., 40: 139-154.57

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 59 - 63SHORELINE STABILIZATION IN ONSLOW BAYHugo Valverde & Orrin H. PilkeyProgram for the Study of Developed ShorelinesDuke University Department of GeologyDurham, NC 27708The shoreline along Onslow Bay is retreating landwardtoward an ever-increasing number of beach front ( buildings.Naturally, beach front property owners wish ( to preservetheir buildings. There are basically 3 ways ( in which theymay do this: (1) move buildings back, (2) armor the shorelinewith seawalls and/or groins, or ( (3) nourish the beach.Of course, there are advantages and disadvantages toeach of these approaches. The retreat option is the ~ best wayto preserve the recreational beach but could i be costly andcertainly is politically difficult. This is because people whoown beach front property tend to be politically influentialand are unwilling to move or demolish their structures.Armoring the shoreline is the best way to preserve buildingsbut this approach results in degradation, and even disappearance,of the recreational beach. For this reason, North Carolinaprohibits all shoreline armoring with the exception ofsandbags. Beach nourishment "improves" the quality of therecreational beach but is very costly and temporary as thelong history of beach nourishment at Carolina Beach andWrightsville Beach demonstrates.BEACH NOURISHMENTBeach Nourishment in Onslow Bay IA surprising number of beach nourishment projectshave been carried out along the Onslow Bay shoreline. Wefound a total of 80 separate pumpings on 11 barrier ( islands,a number that is probably incomplete. All these nourishmentprojects are at least partially federally funded with the exceptionof 4 pumpings on Figure 8 ( Island, a private island notinterested in allowing the ( general public access to a federallyfunded beach.By far the most significant nourished beaches along thisshoreline reach are Wrightsville Beach and Carolina Beach.Both have been steadily nourished with federal funding since1965. These beaches are among the nation's most nourishedbeaches as measured both by frequency of sand applicationand cumulative volume of sand per mile (Pilkey and Clayton,1987). Plots of cumulative sand volumes for Carolina andWrightsville beaches (Fig. 1) indicates that the rates of sandloss from the beaches are more or less constant and thatCarolina Beach has required more sand than WrightsvilleBeach. The fact that sand volume requirements do notdecrease with time indicates that offshore sand from thenourished beach plays no role in beach durability and nourishmentsand is being permanently lost from the profile. Thisis contrary to what modern engineers tell us, which is thatsand lost from the beach remains offshore in the profileaffording storm protection and reducing long term sand volumerequirements.There are 2 kinds of federal beaches; (1) shore protectionprojects best exemplified by Carolina and WrightsvilleBeaches and (2) beach disposal projects. The former arebeaches "designed" by the Corps of Engineers, usually largeones, intended to mitigate the impact of storms. However,the storm damage mitigation justification to fund nourishedbeaches is a mere legal formality. Rather than storm protection,most communities are more concerned with maintenanceof the all-important recreational beach which brings inimportant revenue to the local economy.Beach disposal projects consist of dumping dredgespoil, usually from channel or intracoastal waterway maintenanceprojects, on nearby beaches. Such beaches are notstudied or designed with engineering models, but there isbasically no difference in the application of the sand to thebeaches between shore protection and beach disposalprojects. No predictions are made by the Corps concerninglifespan of beach disposal projects. Disposal projects alsotend to be quite small and are usually free to the local community.An example of a very large beach disposal project isFort Macon/Atlantic Beach. Approximately 20% of the totalreplenishment sand placed on the Onslow Bay Shoreline canbe found here, paid for entirely with federal funds.North Topsail shores has received at least two small disposalbeaches (Table 1 ) in spite of the fact that the communityis part of the Coastal Barrier Resources Act system. Thisis a program that prohibits any federal expenditures on designatedbarrier islands as a means of discouraging developmentin dangerous areas.Nourished beaches classified as shore protectionprojects are designed by individual Corps districts usingdeterministic models of beach behavior. We (Young, et al.,1995; Pilkey, et al., 1993) believe the models do not workbecause assumptions used by the Corps are too simplisticand generalized to accurately reflect natural beach behavior.In addition, the random occurrence of storms requires aprobabilistic view of beach behavior and should produceanswers in sand volume and cost estimates with error bars.Current practice, however, does not produce estimates thatinclude error bars. This can clearly be seen in figure 1 whenone compares actual and predicted cumulative sand volume.59

HUGO VALVERDE & ORRIN H. PILKEYFigure 160

SHORELINE STABILIZATION IN ONSLOW BAYTable 1. Beach Nourishment History of Onslow Bay Shoreline. An asterick* denotes a maintenance dredging project with beachdisposal. Projects without an asterick are “designed” shore protection projects. Sources: Pilkey and Clayton (1989), records of theU.S. Army Corps of Engineers, Wilmington District, and North Carolina Division of Coastal Management.61

HUGO VALVERDE & ORRIN H. PILKEYAll of the predictions depicted underestimated sand volumerequirements due to "unexpected' storm events. Storms, however,should not be considered unexpected, especially on thebarrier islands of North Carolina. Engineers need to recognizestorms as geologic agents, existing as part of a verydynamic system, that can be expected to occur and should beplanned for.SHORELINE ARMORINGRelative to most states, the North Carolina shoreline isonly lightly armored. According to Pilkey and Wright (1991)approximately 5% of the state's developed shoreline isimpacted by shoreline armoring. This compares with 25% ofthe developed shoreline of South Carolina, 45% of east Florida'sdeveloped shoreline and 50% of the developed NewJersey shore.Seawalls are designed to prevent shoreline retreat and inthe case of very large seawalls, absorb storm wave impact.Along the South Carolina shore, virtually every seawall inthe Hurricane Hugo impact area was overtopped by wavesand storm surge. These seawalls, however, were low andwere designed more to prevent shoreline retreat than to preventwave attack on buildings.Seawalls destroy beaches. This was once a controversialstatement but not any longer. Such beach degradation mayoccur in 3 ways: placement, passive, and active loss. Placementloss occurs when a wall is actually constructed seawardof the high tide line. Passive beach loss occurs as a result ofplacement of any fixed object (seawalls, highways, buildings)at the landward side of the beach. Eventually (2 to 4decades in Onslow Bay), the beach backs up against the walland becomes narrower. Active beach degradation occurs as aresult of the interaction between the surf zone and a seawallbut little is known of such processes. Most of the controversyconcerning the impact of seawalls on beaches centers on theactive degradation mechanisms. However, where there is nocontroversy is the fact that once seawalls protrude into thesurf zone, they can behave like groins and actively causebeach retreat on adjacent beaches through the reduction ofsand in the longshore transport system.Two major seawalls exist in Onslow Bay. One in AtlanticBeach on Bogue Banks, was constructed shortly after andin response to the 1962 Ash Wednesday Storm. Gradualbeach narrowing occurred in front of this wall leading to theCorps' beach nourishment projects involving sand pumpingfrom Beaufort Inlet and Morehead City harbor, locatedbehind the island. The second large wall was constructed in1995 for protection of the confederate earthworks of FortFisher, south of Carolina Beach. The Fort Fisher seawall wasthe first variance to North Carolina's anti-armoring regulationswhich were put into effect ten years earlier, in 1985.This wall will eventually increase shoreline erosion ratesboth to the north and to the south. One of the justificationsfor this wall is prevention of inlet formation across FortFisher which could cause sedimentation in the WilmingtonHarbor navigation channel. To us, this is a poor justificationfor a seawall because, like all other east coast inlets formedduring storms over the last 40 years, anew inlet at Fort Fisherwould immediately be filled in by State or Federal agencies.Using the inlet formation argument on all North Carolinaislands could justify the armoring of most beaches, evenundeveloped ones.Besides the Fort Fisher and Atlantic Beach seawalls,only a few others exist along Onslow Bay. These are mostlyisolated walls or bulkheads in front of individual buildingson Kure Beach, Topsail Island, and Bogue Banks. A fewsandbag revetments exist here, permitted by the State inorder to "temporarily" protect exposed buildings.RELOCATIONThe conclusion of a white paper (Howard et al., 1985)produced by a group of coastal scientists, planners and engineerswas as follows: Sea level is rising and the Americanshoreline is retreating. We face economic and environmentalrealities that leave us two choices: (1) plan a strategic retreatnow or (2) undertake a vastly expensive program of armoringthe coastline and, as required, retreating through a series ofunpredictable disasters.The relocation alternative has been "practiced" on mostof Onslow Bay's islands, especially in the early days whenmoving shorefront buildings was a common event. Still,many more houses have been destroyed by storms thanmoved back by their owners.Communities where beach front buildings have beenrelocated because of the erosion threat include BaldheadIsland, Topsail Island, and Bogue Bank. The island with thegreatest need for the relocation alternative is Topsail, wherehundreds of dwellings, most built decades ago, are nowperched on the brink of disaster. Because of this, most of thedevelopment on Topsail was recently destroyed by hurricaneFran. Along the Long Bay shoreline to the south, LongBeach and Holden Beach also have many buildings veryclose to the surf zone.REFERENCESHoward, J.D., Kaufman, W., and Pilkey, C.H., 1985. National Strategyfor Beach Preservation. in proceedings of Second SkidawayInstitute of Oceanography Conference on America's ErodingShoreline, 11 p.Pilkey, C.H. and Clayton, T.D., 1987. Beach Replenishment: TheNational Solution? ..in proceedings of Coastal Zone '87, pp.1408-1419.Pilkey, C.H., and Wright, H.L., 1988. Seawalls Versus Beaches,Journal of Coastal Research. Special Issue 4, pp. 41-64.Pilkey, C.H., and Clayton, T.D., 1989. Summary of Beach ReplenishmentExperience on U.S. East Coast Barrier Islands. Journal62

SHORELINE STABILIZATION IN ONSLOW BAYof Coastal Research, 5, pp. 147-159.Pilkey, O.H., Young, R.S., Riggs, S.R., Smith, A.W., Wu, H.W., andPilkey, W.D., 1993. The Concept of Shoreface Profile of Equilibrium:A Critical Review. Journal of Coastal Research, 9, pp.255- 278.Young, R.S., Pilkey, O.H., Bush, D.M., and Thieler, E.R., 1995. ADiscussion of the Generalized Model for Simulating ShorelineChange (GENESIS). Journal of Coastal Research, 11, pp. 875-886.US Army Corps of Engineers, Wilmington District, 1962. WrightsvilleBeach, North Carolina. House Document No. 511, 87thCongress, 2nd Session. Washington D.C.: U.S. GovernmentPrinting Office, 115p.US Army Corps of Engineers, Wilmington District, 1964. CarolinaBeach, North Carolina, Shore and Hurricane ProtectionProject, General Design Memorandum, 19p.US Army Corps of Engineers, Wilmington District, 1983. FeasibilityReport and Environmental Assessment on Shore and HurricaneWave Protection, Wrightsville Beach, North Carolina,45p.63

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 65 - 72FORT FISHER REVETMENT PROJECTWilliam A. DennisUS Army Corps of Engineers, Wilmington DistrictCoastal, Hydrology and Hydraulics SectionPO Box 1890Wilmington, NC 28402-1890ABSTRACTFort Fisher, North Carolina's most visited historic site,has been subject to shoreline erosion for more than a century.In 1996, the construction of a major rubblemound revetmentwas completed to prevent further loss of the historic property.This $4.6 million 3,040-foot-long structure consists of3-ton granite armor stone placed over marine limestone beddingand underlayers. The project background, includingmajor historic shoreline changes and past shore protectionefforts leading to the selected plan, is discussed. Also, thehydraulic design details, shoreline impacts of the structure,along with a description of the revetment construction arepresented.INTRODUCTIONFort Fisher was built by the Confederacy to guard theentrance of the Cape Fear River allowing vital supplies toreach the port of Wilmington during the Civil War. On a coldJanuary day in 1865, Union forces assembled a massivenaval attack capturing the fort following a bloody day offighting and some 1 ,800 casualties. On a hot July day in1996, more than 131 years later, ground was broken for constructionof a large stone revetment in another battle of sorts,this time against storm attack and beach erosion, that hadclaimed more than one-half of the original fortifications. Theconstruction of the 3,040-foot-long structure followeddecades of planning by local, State and Federal agencies,along with several prior shore protection efforts of limitedscope and effectiveness.The project site is located along the southeastern coastof North Carolina in southern New Hanover County, on apeninsula that separates the lower Cape Fear River from theAtlantic Ocean (Figure 1). The historic site is immediatelysouth of Kure Beach and is located approximately 20 milessouth of the city of Wilmington. The revetment also affordsprotection to a portion of US Highway 421 that passesthrough the project area providing access to the North CarolinaAquarium, the Fort Fisher State Recreation Beach, andthe Fort Fisher/Southport Ferry.BACKGROUNDFigure 1. Location map of Ft. Fisher project area.The original fortifications consisted of a series of interconnectedearthenmounds stretching east-west one- third ofa mile across the peninsula and then generally north-southalong the ocean frontage for about one mile. Beneath themounds was a complex system of interior bunkers andbombproofs that protected men, ammunition and suppliesduring bombardment. The present site covers 264 acres andis administered by the North Carolina Department of CulturalResources. Fort Fisher is North Carolina's most visitedState historic site and in 1962 was the first State site to bedesignated as a National Historic Landmark. This is thehighest designation given by the Federal government in recognizingand encouraging preservation of the nation's importanthistoric properties. The current historic site includes theremaining earthenwork fortifications (part of which has been65

WILLIAM A. DENNISrestored), a museum/visitor center, a picnic area along thebeach front, and a memorial and ocean view parking at "BattleAcre."MAJOR SHORELINE CHANGESWhen Fort Fisher was constructed in the 1860's therewere two entrances into the Cape Fear River. The fort wasbuilt along the northern shoulder of the northernmostentrance which was known as New Inlet. The southernentrance was located at the site of the present Cape Fearentrance near Southport, NC. Since the time of the Civil War,the Fort Fisher area has experienced major shorelinechanges. These have been the result of man's action, as wellas those induced by natural causes."The Rocks"The first major change occurred as a result of the constructionof a rock dam across New Inlet beginning in the1870's. This work, known locally as "The Rocks" was undertakento prevent a persistent shoaling problem in the CapeFear River. The dam prevented tidal exchange between theriver and the ocean at this location and forced the main tidalflow south through the primary river entrance near Southport.The construction resulted in significant change in theinlet complex. The much reduced tidal prism allowed for theonshore migration of sand from the ebb tidal shoal and thedevelopment of a southward migrating sand spit into theinlet. The outcome of these processes was the modificationof the shoreline from one with a large seaward bulging planformalong the shoulder of the inlet, to a more linear alignmentalong the fort. Figure 2 depicts these major shorelinechanges, comparing the modern day shoreline (1992) withthat which existed in 1865.These changes had both positive and negative impacts.On the positive side, navigation was improved in the riverand miles of new beach formed south of the fort. Today, thisnew strand comprises much of the Fort Fisher State RecreationArea. On the negative side, the shoreline realignmentresulted in severe erosion in the area of the earthenmoundfortifications. For example, since 1865 the shoreline haseroded about 1,200 feet in the vicinity of the fort, for a longtermaverage erosion rate of 9.5 ft/yr.Coquina OutcropsThe second major influence on the shoreline in theimmediate vicinity of the fort has been the emergence of naturalcoquina rock formations just offshore and to the north ofthe fort (Moorefield, 1978). For at least the last 60 years, therock outcrops have modified sediment transport patternsalong the shoreline fronting the fort, causing accelerated erosionalong this reach. The net longshore sediment transportfor the area is from north to south, placing the project sitedowndrift of the coquina rock outcrops. The characteristicshoreline response to the emerging rock formations is thedevelopment of a large embayment downdrift of the moreerosion resistant rock formations. Unfortunately, much of thefort itself falls within the shoreline embayment caused by theoutcrops, resulting in accelerated erosion of the historic fortifications.SHORE PROTECTION EFFORTSAlthough numerous studies addressing the erosion problemat Fort Fisher have been accomplished over the years,beginning as early as 1931 (US H.DOC 204, 1931), noaction was undertaken until 1955. During this year, theCounty undertook the first of a series of emergency actionsaimed at preventing further erosion of the historic site. Thisaction consisted of constructing two short groins in front ofBattle Acre, following severe erosion that resulted from aseries of hurricanes which affected the area in 1954 and1955. These included Hurricane Hazel which is recognizedas the storm of record for the area. Subsequent shore protectionactions are summarized in Table 1.Table 1. Fort Fisher Shore Protection HistoryThe most significant measures have been the constructionof two revetments prior to the major revetment constructionthat is the subject of this paper. The first of theseinvolved the placement of concrete and brick rubble alongabout 700 feet fronting the Battle Acre area. Over the years,this structure had been maintained through the sporadicplacement of construction rubble. This material consistedchiefly of concrete slabs and masonry debris collected fromstructural demolition. This structure had proved to be fairly66

FORT FISHER REVETMENT PROJECTFigure 2. Map showing shoreline changes between 1865 and 1992 and footprint of the original fortifications.67

WILLIAM A. DENNISeffective in erosion control, mainly through the replenishmentof the rubble material over the last 25 years. The secondmeasure was undertaken by the State in 1970 involvingthe construction of an emergency revetment. This structurewas built of locally available marine shell limestone andextended from Battle Acre northward to the coquina outcrop.During the 1975-76 timeframe, the revetment was breachednear its midlength and began a gradual progressive failureextending both northward and southward from the midpoint.Presently, remnants of the revetment are visible along itslength with the stone having settled into the beachface. Someof this material had to be moved since it was along the alignmentof the 1996 revetment project. The material was repositionedjust seaward of the new work as an added scourprotection measure.SELECTED PLANGiven the importance of protecting the remaining fortificationsand with the effort to do so being beyond the localmeans, a beach erosion control protect was authorized by theUS Congress in 1976. Under this authority, the US ArmyCorps of Engineers, Wilmington District, initiated a planningand design effort addressing a number of alternative shorelineprotection measures (US Army Corps of Engineers,1981) .The alternatives considered were a revetment, beachnourishment, offshore breakwaters, groins, and various combinationplans. From this initial effort, the recommendedplan consisted of a stone revetment to protect the upland portionsof the fort along with a beach fill and groin field tomaintain a beach in front of and to the south of the stonewall.Upon completion of the general design, the projectbecame inactive until the early 1990's due principally to budgetrestraints with State matching funds. At that time, anactive group known as The North Carolina Committee toSave Ft. Fisher organized and was successful in lobbying toget the project reactivated. In the spring of 1993, plans toconstruct the revetment were finalized; however, the beachfilland groin field portions of the plan were deferred basedon cost considerations and environmental issues associatedwith the selected beachfill borrow area.Shore Hardening IssueThe selected plan consists of a rubblemound structurethat extends from the northern property line of the historicsite, near the coquina outcrop, southward beyond Battle Acrewhere US Highway 421 would be protected (See Figure 1 ).The purpose of the structure is to protect the upland behindthe revetment and not to preserve the beach fronting thestructure. This proposed plan was contrary, however, to theNorth Carolina state regulations governing development inthe coastal area. Since the mid-1980's, standards establishedFigure 3. Schematic showing idealized shoreline impacts of theFt. Fisher revetment.by the Coastal Area Management Act prohibit the use ofhard structures in the coastal environment. These rules wereadopted to prevent possible adverse impacts to adjacentproperties or public access that could be caused through theindiscriminate use of hard structures.The rules, however, did not recognize the public benefitsto be realized through the long-term protection of an historicsite of national significance. In this regard, anew rule wasadopted in December 1992, following significant controversy,that would permit a coastal structure to be used to protectan historic site under certain conditions. The followingyear, the Fort Fisher plan was found to be in compliance withthe rule, and a permit was issued allowing the project to goforward.Central to the controversy was the concern over theimpact that the proposed revetment could have on the downdriftbeach south of the site. Future erosion conditions at thesite, with or without the revetment are expected to be dominatedby the presence and continued emergence of thecoquina outcrops. With the construction of the revetment, themajor difference will be that the upland containing theremaining fortifications and adjacent picnic area (area68

FORT FISHER REVETMENT PROJECTbetween Battle Acre and the coquina outcrop) will be stabilized.Prior to construction this area was eroding at an averagerate of 11 ft/yr. The material from this area served as asource of sediment elsewhere in the littoral zone. With therevetment in place, this source is no longer available.Assuming that most of this material fed the downdriftbeaches to the south, the primary impact of the revetmentwill be increased erosion to the south equal to the induceddeficit. This impact is quantified and shown schematically inFigure 3.As indicated on the figure, the picnic area was losingabout 0.25 ac/yr prior to stabilization. The reach downdriftof Battle Acre was eroding at an average rate of 10.2 ft/yrand had experienced an average area loss of about 1.6 ac/yr.This reach extends approximately 7,000 feet south of BattleAcre. Beyond this reach, the erosion rate was found todecrease significantly indicating the southern limit of thedowndrift impact zone that existed prior to the revetmentconstruction. With the revetment in place, the loss within thepicnic area has been eliminated. However, this loss will ineffect be displaced to the downdrift zone. This increases theerosion to 1.9 act yr or 11.8 ft/yr for this zone, whichamounts to a 15% increase in erosion along the downdriftreach. It is noted that this estimate is considered conservativeand probably overstates the increase in construction inducederosion by assuming that all the sediment that was being lostfrom the picnic area served as a source to the downdriftbeach. A portion of this area contained an actively erodinglayer of humate-rich sand. Once eroded, this fine peat-likematerial was most likely transported offshore and out of theactive littoral zone and, therefore, represented a loss to theoverall sediment budget.The predicted 15% increase in erosion associated withthe new revetment construction would be felt along beachesto the south of the structure. This area is public State ownedland as is part of the Fort Fisher State Recreation Area.Therefore, in terms of overall public benefit, the use of therevetment represents a trade- off between the preservation ofhistoric lands versus an increase in the loss of public beach.A condition of the permit requires that if the future erosion isfound to exceed the 15% threshold, appropriate mitigationmeasures will be undertaken.REVETMENT DESIGNThe revetment was designed to stabilize the erodingshoreline and protect against storm wave attack and overtopping(US Army Corps of Engineers, 1993) . The selecteddesign conditions represented those :hat were associatedwith Hurricane Hazel, the storm of record for southeastNorth Carolina. Specifically, these conditions were a peakstorm surge of 10.7 feet above mean sea level and a breakingwave height and maximum wave period of 11.8 feet and 12seconds, respectively. In terms of water level, 'his representsan event with a peak surge slightly greater than the 100-yrbase flood elevation as reported by the Federal EmergencyManagement Agency (1986).The structural design of the rubblemound revetment wasaccomplished primarily through the use of two dimensionalhydraulic model tests conducted at the US Army Corps ofEngineers, Waterways Experiment Station, (Markle, 1982).The tests were performed in a wave flume at a scale of 1 :24,model to prototype. Atypical 48-foot wide section of the prototype(2-foot model section) was subjected to depth limitedbreaking waves and varying storm hydrographs simulatingdesign loading conditions. Additional tests were performedFigure 4. Typical revetment x-section showing various stone layers and position of STA-POD units.69

WILLIAM A. DENNISFigure 5. View of completed revetment looking northward from the Battle Acre area.1.The STA-POD was invented in the 1960’s by Mr. RaymondO’Neill, Concrete Armor and Erosion Consultant,Spring Lake Heights, NJ.in assessing the stability of the structure to conditions bothsubstantially greater than and less than the design condition.A typical x-section of the revetment, as shown in Figure4, consists of three layers of successively smaller size stoneplaced over a synthetic filter cloth. Sand backfill is providedlandward of the stone structure providing a sloping gradebetween the existing ground and the crest of the revetment.The three layers known as the armor layer, underlayer andbedding consist of individual stones with median weights of3 tons, 650 lbs. and 20 lbs., respectively.The crest elevation of the 3,040 foot-long revetment is +13 feet above mean sea level, except at the southern end ofthe structure and Battle Acre. At the southern end, the crestis lowered to + 10 feet to follow the existing topography. Forthe area along Battle Acre, the crest is gradually increased toan elevation + 15 feet, maintaining this elevation along a180-foot reach immediately fronting the remaining earthenfortifications. The base elevation of the revetment is - 3.5feet throughout its length.Another design feature is a horizontal stone apronlocated at the seaward toe of the structure to prevent waveinduced scour from undermining the revetment. The scourapron is anchored by a unique interlocking concrete unitknown as a STA-POD 1 . The STA-POD consists of a cylindricalmain trunk with four stabilizing legs. The legs areslightly longer than the central trunk which allows them tosettle into the supporting stone bed for greater stability. Eachunit is steel reinforced and stands about 7.5 feet tall, has anoverall base width of 12 feet and weighs 5 tons. When positioned,the legs of adjacent units overlap, forming a continuousinterlocking line of defense along the toe of thestructure. In addition, underlayer stone is placed around thelegs of the units further enhancing their stability.PROJECT CONSTRUCTIONThe revetment construction began in June 1995 and wascompleted in January 1996 (Figure 5). The $4.6 million contractwas awarded to Misener Marine Construction Companyof Tampa, FL. The cost was shared 50/50 between State andFederal governments. The work involved transporting andplacing about 70,000 tons of rock. The 3-ton armor stoneconsisted of biotite granite that was obtained from NeversonQuarry located in Bailey, NC, about 125 miles from the site.The underlayer and bedding stone consisted of less densemarine limestone which came from the Martin MaritettaQuarries in Castle Hayne, NC, and Rocky Point, NC, about25 and 35 miles from the site, respectively. All stone wastruck hauled to the project and either stockpiled or offloadeddirectly into the work.The STA-PODs were cast on site in an area located justto the north of museum (Figure 6). Six intricate forms were70

FORT FISHER REVETMENT PROJECTFigure 6. The STA-POD casting area showing the units shortly after removal of the steel forms.fabricated by Helsner Industries, of Tualaton, OR, whichconsisted of eight removable sections for each unit. The reinforcingsteel cages were hand fabricated at the site for positioningin each leg. All rebar was epoxy coated for corrosionresistance. Due to the severe wave loading conditions, veryhigh strength concrete was required having a compressivestrength of 6,000 psi and a minimum unit weight of 150 pcf.Each unit required about 2.3 cy of concrete. Once in productionthe casting operation could produce six units per day,taking about three months to cast the 400 STA-PODSrequired for the project.PUBLIC AMENITIESIn addition to the revetment itself, the project involved anumber of ancillary features to enhance the safety and enjoymentof the visiting public. These features include a 2,770-foot-long asphalt walkway, two timber overlooks, beachaccess stairs over each end of the revetment and native beachlandscaping. Work on these features began following theconstruction of the revetment and was completed in June1996 marking the end of the construction contract.POST-CONSTRUCTION MONITORINGWith the construction of a major coastal structure it is ofinterest to document the impact that such a structure mayhave on the adjacent shorelines. In this regard, a monitoringprogram has been initiated to measure the post-constructionshoreline response and then compare this with shorelinechanges that existed prior to construction.The impacts of the revetment on the adjacent shorelinesare being monitored using beach profile surveys and aerialphotography. The beach profile surveys are being performedtwo times per year, during March and September. Surveys atthese times of the year will generally capture the range ofshoreline changes, with the surveys following the erosionalwinter storm season and the accretional summer season. Theaerial photography is taken during the March survey periodso the changes from the photos can be correlated with measuredchanges from the beach profiles. In addition, the photographyprovides spatial continuity between the individualbeach profile locations.The monitoring area covers a 20,000-foot reach ofshoreline, extending 3,000 feet northward and 17 ,000 feetsouthward from the Fort Fisher Museum. This involves theperiodic survey of 50 beach profiles generally spaced 500feet apart, except those near the structure which have spacingsof 200-300 feet. The beach survey cover the onshorearea (from a stable upland bench mark to wading depth) withoffshore portions of the profile (approximately to the 30-footcontour) being taken on alternate surveys, i.e. Septemberonly. In addition to the beach surveys, 30 control pointslocated on the revetment are being surveyed semi-annuallyto monitor settlement and stone movement.To date, a base-condition survey, a during construction71

WILLIAM A. DENNISsurvey, and two post-construction surveys have been accomplished.The first in a series of monitoring reports, documentingthe findings over the initial 2-years, is scheduled forpublication in the spring of 1997.CLOSINGYears of planning, study, and design culminated thisyear with the construction of the revetment for shore protectionof Fort Fisher. This effort was accomplished by individualsat all levels of government, as well as concerned historicpreservationists. With construction of the revetment, whichis designed to withstand a rather rare storm event, at least 50or more years of protection should be provided to the fortwithout significant maintenance or rehabilitation.REFERENCESMarkle, Dennis, G., 1982, Revetment Stability Study, Fort Fisher,AD A 123 754, State Historic Site, North Carolina, TR HL-82-26, US Army Corps of Engineers, Waterways Experiment Station,Vicksburg, MS.Moorefield, Thomas, P., 1978, Geological Processes and History ofthe Fort Fisher Coastal Area, North Carolina, Master Thesis,Department of Geology, East Carolina University, Greenville,NC.US Army Corps of Engineers, 1981, General Design Memorandum,Phase 1, Fort Fisher and Vicinity, US Army Corps ofEngineers, Wilmington District, Wilmington, NC.US Army Corps of Engineers, 1983, Fort Fisher, North Carolina,Phase II General Design Memorandum Supplement, US ArmyCorps of Engineers, Wilmington District, Wilmington, NC.US Congress, House of Representatives, 72nd Congress, 1 st Session,Document No. 204, 1931 , Fort Fisher, NC.Federal Emergency Management Agency, 1986, Flood InsuranceStudy, New Hanover County, North Carolina, UnincorporatedAreas, Federal Emergency Management Agency, Washington,DC.72

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 73 - 107ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NORTH CAROLINAREGIONAL OVERVIEWWilliam J. ClearyDepartment of Earth Sciences andThe Center for Marine Science ResearchUniversity of North Carolina at WilmingtonWilmington, North Carolina 28403Orrin H. PilkeyGeology DepartmentDuke UniversityDurham, North Carolina 27708The Georgia Bight is the 1,200 km coastal reachextending from Cape Hatteras NC to Cape Canaveral, Fl.The coast of the bight consists of a nearly continuous chainof barrier islands situated on the tectonically stable trailingedge of the North American Plate. The North Carolinacoastline, on the north flank of the bight, consists of asequence of large capes and associatedshoals, barrier islands, spits, and occasionalheadland areas. A natural division ofthe North Carolina coastline occurs near CapeLookout (Fig. 1). North of this Cape, the islandsare separated from the mainland by relatively wideopen water lagoons and sounds, that back the barrierislands are narrow and nearly filled withmarsh.The continental shelf segment between CapeLookout and Cape Fear is referred to as Onslow Bay.Only small coastal plain rivers empty into thiscoastal segment which probably was also the caseduring most of the late Pleistocene. The larger riversthat originate in the Piedmont province emptyinto Raleigh Bay to the north and Long Bay tothe south. As a consequence, this shelf areaprobably has the lowest sedimentation rate onthe US East Coast continental shelf. The thinshelf sediment cover as residual, meaningthat it is derived from underlying ancient sedimentsthat frequently crop out on the shelf.Other shelf areas of North Carolina are mainlycovered by relict sediments, left stranded afterbeing deposited at lower sea levels.The 13 barrier islands that comprise the 150km coastline between Cape Lookout and CapeFear have a wide variety of physiographic forms. rangingfrom overwash- dominated narrow barriers to wide barrierswith massive dunes and no washovers. The approximatedivision between the two morphologic classes occursbetween Browns Island and Onslow Beach (Fig. 1 ).In this area a submarine headland composed of TertiaryFigure 1. Location map depicting the barriers, inlets and fieldtripstops. R=Regressive barrier segment, T=transgressive barriers.73

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 2. Cross-sections of Onslow Bay barriers.A. Regressive barriers are sand rich and contain 15-25 times more sand per unit length of coast than transgressive barriers.B. Transgressive barriers are low, narrow and prone to frequent overtopping.limestone and sandstones. forms a small bulge in the coastlinethat separates the relatively stable, sand rich regressivebarriers to the northeast from the transgressive eroding barriersto the southwest (Fig 2A, 2B). At Fort Fisher/KureBeach, a short stretch of mainland shoreline exists in theform of a subaerial headland composed of Pleistocene sandstonesand coquina. This forms the southern boundary of thesand poor transgressive segment. A narrow spit extendssouthward from the headland to the Cape Fear Foreland.The longest of the barriers along the Onslow Bay shorelineis Bogue Banks, 45 km long; Lea Island is the shortest inthe chain, approximately 2 km long. Lagoons are widest (4.1km) in the north, behind Bogue Banks, and generallydecrease and finally disappear where the islands have overriddenthe subaerial headland at Fort Fisher/Kure Beach. Thenorthern lagoons, principally Bogue Sound, are largely shallow,open, and free of vegetation. By contrast, tidal marshesgenerally have infilled the southern lagoons. Elevations onthe islands range from less than three meters on MasonboroIsland to more than 10 m above MSL on Bear Island.These microtidal to low mesotidal barrier shorelines(Hayes, 1979 and 1994), have a mean tidal range of aboutone meter. The direction of wind approach fluctuates annually.During the spring and summer the winds are from thesouth and southwest, while during the winter, north andnortheast winds will prevail. The coastline just south andwest of Cape Lookout is more protected from northeaststorms than the area north of Cape Fear. All sections arehighly vulnerable to hurricanes approaching from the southand east.Inlets vary from wide, deep, stabilized and maintainedinlets such as Beaufort Inlet (Fig. 3A), separating Bogue and74

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 3. Inlet types. See figure 1 for locations.A. Beaufort Inlet is a large modified stable system thatserves as the port of entry for Morehead City Harbor. The1956 photograph shows large sand bars migrating and weldingonto the Shackleford Banks shoulder.B. Mason’s Inlet is a small migrating system that is the centerof controversy. It is slated for relocation. The inlet bordersWrightsville Beach and Figure Eight Island.Shackleford Banks, to narrow, shallow, shifting inlets such asMason's (Fig. 3B), separating Wrightsville and Figure EightIslands. Human impacts on the islands vary from extensivedevelopment on portions of Wrightsville Beach, BogueBanks, and Topsail Island to uninhabited islands such asShackleford, Coke and Bear Islands.ORIGIN OF BARRIER ISLANDSBarrier Islands make up about 10% of the world's shoreline.Chains of barrier islands (3 or more inlets) front threepercent of the coasts of the world. Because barrier islandsare virtually the most important coastal type in North America,they are perhaps the most intensely studied coastal featuresin the world. All barrier islands have four parameters incommon: a rising sea level, a gentle lower coastal plain surface,a sand supply large enough to build islands and sufficientwave energy to move sand. Almost all barrier islandchains are found on the trailing edges of continents althougha few chains (eg. the Pacific Coast of Colombia and the CopperRiver Delta, Alaska) occur on leading edges where avery large supply of sand exists. The Florida shoreline borderingthe northeast corner of the Gulf of Mexico has no barrierislands because the wave energy is too low to movesignificant amounts of sand.The fundamental reason that barrier islands exist is that astraight shoreline, oriented in such a way that longshoremovement of sand is minimized, is astable shoreline configuration.Given an unconsolidated shoreline with waves comingfrom the same direction, erosion and depositionalpatterns would eventually result in a shoreline orientationparallel to incoming wave crests. This, of course, never happensin nature because incident waves come from manydirections. The barrier islands effectively straightened out avery irregular shoreline formed when the lower coastal plainwas flooded by a rising sea level.When sea level began to rise 18,000 years ago, the rivervalleys were flooded and ridges between the valleys becameseaward protruding headlands. Wave erosion of the unconsolidatedheadlands furnished sand to form spits extendinginto the deep water across the throats of the newly formedestuaries. Sand from the eroding headlands formed spitsrather than being deposited inland along the shorelines of theestuaries. These spits were deposited when waves refractedaround the entrance to the estuaries, thus loosing energy andthus a bulk of the suspended sand load. Thus it can be saidthat another fundamental reason that barrier islands exist isthat waves loose energy when they are refracted by landmasses.Once the spits form they are eventually segmented intoislands by the formation of new inlets during storms. Simultaneously,the rising sea level flooded the region behind theridges of sand dunes and overwash fans formed along theopen ocean shoreline, in effect lengthening the spit. Once theislands are formed through a combination of spit breachingand backbarrier flooding during a time of rising sea level, anentire new set of processes takes over. The barrier islandsbegin to migrate apace with the rising sea level.The process of island formation does not occur duringtimes of lowering sea level and seaward advancing shorelines.No estuaries exist to promote the formation of spitsand no particular advantage is gained or fulfilled by straighteningan already regular shoreline. Rising sea level is the keycomponent of barrier island formation and evolution.BARRIER ISLAND MIGRATIONOnce formed, the islands are believed to have migratedacross the continental shelf apace with the rising sea level.During times of particularly rapid sea level change the75

WILLIAM J. CLEARY AND ORRIN H. PILKEYislands may have existed as spits or they may not haveexisted at all. Sand supply may have been the key factor inisland survival. A large sand supply would aid in island survivalin a rapidly rising sea level. A small sand supply wouldlead to the demise of the islands under the same circumstancesof sea level change. Since Onslow Bay has a verysmall fluvial sand supply and a very thin sediment cover, itcan be assumed that during significant parts of the Holocenetransgression, no barrier islands lined the coast. The evidencehere, and on other continental shelves, that barriers didat least periodically exist at lowered Holocene sea levels isthe presence of back barrier lagoon deposits and the occurrenceon the shelf of filled, discontinuous channels believedto be ancient inlets.Barrier island migration occurs through the combinedprocesses of open ocean shoreline retreat and island wideningthrough the landward extension of the back barrier environmentof the island. The net result is an island that moveslandward and upward in response to sea level rise, a ratheramazing natural phenomenon.The open ocean shoreline retreat, more commonlyknown as shoreline erosion, commonly occurs as a result ofseaward removal of sand as a result of storms. Island wideningmay occur several ways including incorporation of tidaldeltas, formation of overwash fans extending into the backbarriersound and the formation of wind flats. Incorporationof tidal deltas (further discussed below) occurs when theflood tidal delta is abandoned because of inlet migration orinlet closing. The former subaqueous shallow sand bodybecomes part of the island by subaerial buildup thru saltmarsh, dune and storm overwash sediment accumulation. Ifan inlet is migrating, the flood tidal delta may becomeattached to the island and widen it along the entire path ofinlet movement.Island widening by tidal delta attachment is a relativelyslow process which often affects only short stretches of theisland. The southernmost two miles of Bodie Island, alongthe Northeast Outer Banks, has been widened since 1843 bythe incorporation of the migrating tidal delta of Oregon Inlet.On the other hand, more rapid, efficient and widespreadisland migration can be achieved by storm overwash across anarrow island. Such overwash can simultaneously widen andin instances elevate the island. During virtually every significantstorm, the 13 km long Masonboro Island has migrated.On Masonboro overwash fans readily extend into the adjacentsalt marsh behind the island, extending the backbarriershoreline in a landward direction. For efficient migration bythe overwash mechanism, an island must be relatively narrow,perhaps 100 m wide or less, depending upon stormmagnitude and frequency.The incorporation of wind blown sediments from theisland into overwash fans or salt marshes in the back barrierenvironments is probably a significant component of theisland migration mechanism in North Carolina. In Texas,especially on Padre Island, wind flat extension of the backbarrier shoreline is the principal mechanism of island widening.Barrier Island migration on the North Carolina Coastprobably in large part came to a halt 3 to 4,000 years agowhen sea level rose close to its present position. Since thattime some of the islands have widened by the incorporationof beach ridges on their open ocean sides.. Such progradedislands are classified as regressive. Islands that remain in astratigraphic mode indicative of island migration are classifiedas transgressive. Bogue and Shackleford Banks areregressive islands. Topsail and Masonboro Islands can beconsidered to be transgressive barriers.Currently a very common behavior of undeveloped barriersworld wide, is island narrowing. That is, shorelineretreat is occurring on both sides of the islands. ShacklefordBanks is a good example of this phenomenon. One interpretationof this behavior pattern is that the islands are respondingto the current sea level rise and that island narrowing is aprecursor of island migration which will occur when theislands are sufficiently narrow for efficient cross-island overwashto occur.COASTAL PROCESSESThe natural processes operating to maintain the shape ofthe barriers along the Onslow Bay portion of North Carolina'sshoreline are the same as they are elsewhere along theworld's coasts, although the rate and dominance of the processesmay differ. In addition to sea level oscillations and littoraldrift, three additional processes affect the shape andpositions of the barrier beach shorelines. These processesinclude inlet formation, migration and closure, oceanic overwash,eolian transport, and influence of underlying geologicframework. In addition to these natural processes, man'sinfluence has had and will continue to have a negative effecton the islands.Oceanic OverwashOceanic overwash characterizes barrier beaches withlow tidal range and moderate to strong wave climate in a climaticzone of intense storms. Overwash is the processwhereby sediment from the lower portions of the activebeach zone are transported through breaches in the dune systemto the back barrier environments during periods of stormsurge and increased wave activity. These environmentsinclude the grasslands, marsh and open lagoons. Sand suppliedto the island during major storms is used to maintainelevation and volume as the island retreats landward.The depth of the overwash penetration (i.e., ocean tomarsh distance) is determined by an array of site- specificcharacteristics such as island width, height, gradient andintensity of the storm (tidal surge, etc.). Overwash plays an76

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCimportant role in island migration and the overall sedimentbudget along the barriers. Overwash is also a major impedimentto transportation on some low lying barriers such asPea Island and Topsail Island. It can even be a threat to lifewhen it prevents evacuation of storm threatened communities.InletsInlets form during storms when trapped soundside waterovertops the narrowest and lowest areas of the barriers. Analysisof historic charts indicates there has been a generaldecrease in the number of inlets along the North Carolinacoast during the past 300 years. During the seventeenth century,as many as seven additional inlets occurred betweenCape Lookout and Cape Fear. The reason for this decrease isopen to speculation but is probably related to storm cyclesand the fact that the lagoons have infilled and hence have areduced potential tidal prism.Inlets vary in respect to size, tendency to migrate, andthe number of years each remains open. One of the contemporaryinlets, New Topsail Inlet, opened prior to 1738 andhas migrated in a southwesterly direction. Bogue Inlet, oneof the largest and most stable of the contemporaneous inletsin the Onslow Bay section, has been open and more or less inthe same location since before the arrival of the first Europeansin the 1580's.Following inlet closure, the flood tidal delta whichoccurs on the lagoon side of these inlets is preserved. Typicallythe flood tidal delta in microtidal regions is larger thanthe corresponding offshore ebb tidal delta. In the wide openwater lagoons backing the Outer Banks, the tidal deltas arewell-developed and commonly well-preserved. Some extendas much as 8 km into the adjacent sound. The flood tidaldelta located on the landward side is an important inletrelatedsand body for three reasons: 1) subsequent attachmentto the barrier adds volume and elevation, 2) after inletclosure, it forms a substrate over which the barrier migrates,and 3) it is a mechanism for infilling the lagoon. Thesupratidal and intertidal portions of the abandoned tidal deltabecome colonized by salt marsh grasses. This combinationof processes (migration and closure) effectively widens theisland as the inlet migrates downdrift.Many relict flood tidal deltas are readily discerniblealong the North Carolina Coast (Fisher, 1962). In the northernpart of the state where the sounds are large and consist ofopen water, the principal geomorphic evidence is the vegetatedflood tidal delta. In southeastern North Carolina, small,narrow elongate islands are found in the partially marshfilledlagoons. These lagoon islands, indicative of formerinlet locations, commonly parallel the main tidal channel, areusually vegetated, and may reach elevations of up to 3 m.Development of these features occurs in a zone where sandsfrom the constricted flood tidal delta overtop the marsh duringunusually high wave activity. Continued migration of theinlet leads to development of additional islands and subsequentpreservation of earlier formed features (Cleary andHosier, 1979).Eolian ActionA variety of depositional and erosional forms associatedwith wind are observed along this segment of North Carolina'sshoreline. These include small mobile sand sheets,medanos, and a variety of vegetated dunes, including naturalforedunes, parabolic dunes, and man-made stabilized dunes.The nature and integrity of the dune system is dependentupon sand supply, island orientation, dominant wind direction,the extent of vegetation cover, and man's historicalinfluences.Factors which determine the dune morphology observedon the barrier islands include the density of vegetation; thewind regime; the degree of stabilization and the time sincethe most recent disturbance; the availability of sediments;and the position of the shoreline.Islands or portions of islands where washovers are frequent,recent, or chronic, possess either no dunes or scatteredforedunes which generally do not form continuous ridges.An example of this dune morphology can be seen on portionsof Onslow Beach and Topsail Island.Where overwash has occurred in the past, yet recoveryhas taken place, usually a single foredune ridge exists. Thismorphology can be observed along Topsail Island and CarolinaBeach. These barriers are often narrow and susceptibleto breaching during storms.Several islands have a history of progradation resultingin the development of multiple parallel dune ridges. Theshoreline is generally stable and possesses a dense vegetationmantle which is primarily responsible for maintenanceof the structure of the dune system since its formation. Muchof Bogue Banks possesses this dune morphology.Where, for one reason or another, the vegetation mantlehas been broken open or destroyed, massive dunes result.Generally, all or the majority of the former beach ridge dunesystem is destroyed and massive mobile dunes (medanos orparabolic) are formed. Bear Island and Browns Island exhibitthis type of dune morphology. Portions of these dune fieldsare presently partially vegetated.BARRIER SHORELINE OVERVIEWShackleford BanksShackleford Bank is an east-west trending island separatedfrom Cape Lookout by Barden Inlet. Shackleford Bankwas recently attached to Cape Lookout, however, a severehurricane in 1933 opened Barden Inlet and Corps of Engineerdredging has maintained the channel since.77

WILLIAM J. CLEARY AND ORRIN H. PILKEYwestward during the past 30 years. Since 1947, the island hasextended more than 1000 m. This region is characterized bylow scattered dunes generally in an arcuate or 'recurved' pattern.Fresh and brackish marshes occupy the low swalesbetween arcuate dune ridges.Unlike Core Banks, Shackleford Bank has had a historyof habitation. In the mid-1800's, the town of Diamond Citywas located on the eastern end of the island. The inhabitants,numbering a maximum of 500, were fishermen and whalers;the island was used as a lookout for whales passing CapeLookout. Hurricanes in 1899 devastated the island and DiamondCity was abandoned. Remaining buildings werebarged to Morehead City and Harkers Island. Today CoreBanks and Shackleford Bank are part of the Cape LookoutNational Seashore. The seashore development plan calls formaintenance of the islands in a near-wilderness condition.Bogue BanksFigure 4. Bogue Banks aerial photographs (1974).A. Forested beach ridges are typical of this regressive barrier.Parabolic dunes disrupt the dune line. Tidal marsh forms onthe flood delta material.B. An open water lagoon along much of the central section ofthe island indicates a lack of inlet activity.Before the turn of the century, the western half ofShackleford Bank was heavily forested. A series of duneridges indicating previous progradation was evident. Muchof the island was probably similar to Bogue Banks as it istoday. A combination of severe hurricanes, overgrazing byferal livestock, and human disturbance of the forest resultedin the disintegration of the vegetation mantle in the late1800's and the subsequent migration of the dunes across theisland. On the lower elevation, narrower eastern end, Shacklefordis a washover island with a very high (perhaps 10 feetper year) rate of erosion near its eastern tip. On the westernend, the migration of the dunes first slowed and then stoppedbefore the forest was entirely overwhelmed. A 5 km longslipface marks the landward migration limit of the interiordunes on Shackleford Bank. Relict ghost forests were firstengulfed and then re-exposed by the migrating dunes, providingevidence of the magnitude of the dune migrationevent. Large swales or 'slacks' have developed where duneblowouts occurred.The western end of Shackleford Bank has been growingBogue Banks is the longest and widest island in theOnslow Bay section of North Carolina (Fig. 1 ). This may bethe largest volume Holocene island on the US East Coast. Itslarge sand volume is indicative of a large sand supply in spiteof the fact that the Onslow Bay shelf overall has a very thinsediment cover (Fig. 4A).This beach ridge barrier island is approximately 45 kmlong and averages 600 m in width. Unlike the areas to thesouth, the lagoon behind Bogue Banks is generally openwater (Fig. 4B). The lack of significant areas of tidal marshsuggests that inlets have not been active on an island scale inrecent historic times. However, historic maps show isolatedoccurrences of old inlets at several sites. These areas includethe low, narrow sites at Emerald Isle and at Atlantic Beach.At this last locality, the vegetated flood tidal delta of Cheeseman'sInlet (ca. 1850) now forms the site for extensive development(Atlantic Beach).Bogue Banks, located on the low energy limb of theCape Lookout foreland, is morphologically unlike the majorityof islands in North Carolina. It is characterized by anextensive forested beach ridge system with isolated ridgeelevations in excess of 12 m (Fig. 4A). This sequence ofancient dune ridges indicates a period of progradation.Recent studies indicate progradation began 3800 years B.P.(Steele, 1980; Heron, et al, 1984).The island's fronting dune system is largely intact. Multipleor massive dunes are characteristic. Within these areasare sites of blowouts and migrating parabolic dunes. The initiationof these features was presumably due to fires, storms,and man, all of which destroyed the binding vegetation andpermitted remobilization of the sand (Fig. 4A). A few areasdo have a poorly developed dune system. In these regions,the dunes are low, narrow and scarped.Overwash is not an important environmental parameterexcept in those areas where dunes are lacking or poorly78

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 5. Beaufort Inlet line drawings. Line drawings from bathymetric charts depict channel and ebb delta morphologic changes.Dredging of the ship channel has resulted in a seaward growth of the ebb platform and significant volume losses.79

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 6. Aerial photographs of Bogue Inlet.A. 1938. Ebb channel is located on western shoulder of the inlet. A large spit extended westward from Bogue Banks.B. 1994. Channel reorientation and repositioning has occurred several times since 1940. Channel orientation dictates ebb deltasymmetry and erosion patterns on adjacent barriers. Spit growth on Bear Island (left) began after breaching of Bogue Banks spit(see “A”) and the concomitant relocation of the ebb channel.80

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCdeveloped. The sheltering effect of Cape Lookout and theisland's east-west orientation are responsible for this situation.The island is regarded as an excellent site for developmentbecause of its generally high elevations and large volumeof sand. There are, however, several localities which arepoor sites for development. These areas are low and narrowsuch as the Twin Piers section of Emerald Isle. AtlanticBeach is the most modified area due to development. Theseawall and flattened dunes offer little protection fromstorms. Atlantic Beach and other sections of the island havebeen the recipient in recent years of several "free" navigationreplenishment projects.Beaufort Inlet located approximately 9 miles west ofCape Lookout serves as the connection between the AtlanticOcean and Morehead City Harbor, North Carolina's secondmajor port. The inlet is utilized by commercial and recreationalvessels and is one of two inlets in southeastern NorthCarolina which have been modified for commercial traffic.The inlet forms the eastern border of Bogue Banks and separatesthe barrier from Shackelford Banks to the east (Figs.3A & 5A-F).Seismic data indicate the postion of the inlet is controlledby an ancestral drainage pattern that can be tracedacross the inner shelf. Historic maps that date to the earlypart of the seventeenth century confirm the existence of theinlet. Since the Colonial Period the inlet has served as anentry to the port of Beaufort (Stick 1958; Angeley 1982).Beaufort Inlet has remained in relatively the same locationthroughout its recorded history. The large tidal prism associatedwith the Newport and North Rivers that empty into theadjacent sound contribute to the stability of the inlet. A tidalprism of 3.4 x 10 9 cu. ft. has been empirically derived utilizingdata from bathymeric surveys.Prior to 1936, throat characteristics were quite variable.However over the past 60 years, since the channel has beenin a fixed position, the inlet's cross sectional area has fluctuatedvery little, although the inlet's minumum width hasdecreased. During the same period, the average depth of thethroat has increased as the navigation channel was deepenedand widened. As a result, the inlet's aspect ratio (wid) hasdecreased markedly since 1952 as the inlet constricted anddeepened with dredging.Reasonably accurate bathymetric charts are availablefrom 1839 to present and provide a means of tracking thechanges in the inlet, shoal morpholgy and general shorelinemovements. Data from these surveys indicate Bogue Bankshas accreted 70 m in an easterly direction since 1839. Bycontrast, the Shackelford Banks margin has been extended tothe west a net distance of 580 m. The dramatic changes onthe eastern margin of the inlet are even more striking whenone considers that the Shackelford Banks margin was relativelystable for 75 years, and since 1952 has extended 1,325m in an westerly direction. Accretion on both shoulders hasresulted in a constriction of the inlet. These changes over thelast 50 years are primarily related to dredging of the outerbar channel. Since dredging of the fixed channel began, therehas been a deepening and steepening of the profile and agenerally lowering of the ebb platform (Fig. 5A-F).Calculations involving changes in the volume of sedimentstored in the 1854 ebb delta, indicate there was 37.4million cu.m of sand contained in the shoals to depths of 6m. Between 1854 and 1936, the ebb delta volume rangedfrom a low of 35.7 to a high of 43.3 million cu m. Sincemajor dredging efforts began in the mid 1930's the ebb deltavolume has steadily decreased from 36.9 million cu m in1936 to 24.2 million cu m in 1974, a 34.2 % loss. Althoughboth the east and west segments of the ebb delta have lostappreciable volumes of sand, the downdrift eastern lobe haslost 46 % or 8.4 million cu m since 1936. By contrast, thewestern lobe has lost 4.2 million cu m or 22 % during thesame period. Studies are currently underway to determinethe shoal changes since 1976. Preliminary information suggestsa continuing loss of sediment amounting to millions ofcubic meters.A substantial portion of the total loss (12.5 million cum) can be related to dredging activities which average608,000 cu m/y. Tidal flushing of sediment beyond the ebbplatform and into the estuarine system are additional mechanismsthat can account for part of the loss. USACE data suggeststorm events remove substantial quanties of sedimentfrom the ebb delta, transporting it to the inner shelf (USACE1976).Bogue Inlet, a wave-dominated inlet, is one of NorthCarolina's largest inlets and is located at the mouth of theWhite Oak River (Fig. 1 ). Recent seismic studies of theadjacent continental shelf indicate the inlet's position is controlledby the paleochannel of the ancestral White Oak River(Hine and Snyder, 1985). The inlet is relatively stable,although the shallow gorge has migrated within a 2.4 kmzone along Bogue and Bear Islands during the last 200 years.Since 1938, the main ebb channel has changed its orientationand position several times. The consequence of thisreorientation is the landward migrations of extensive packetsof swash bars which weld onto the adjacent beaches (Figs.6A&B). The large spit which extends in an easterly directionfrom Bear Island indirectly stems from the above mentionedcyclical reorientation.Bear and Browns IslandsBear and Brown Islands are 3.5 and 4.5 km long respectively.They average 600 m wide. They can be classified asaltered beach ridge barriers (Fig.7). Large medano-like andparabolic dunes characterize major portions of both islands.The earliest aerial photographs (1938) show the majority ofboth island surfaces were covered by large sand sheets withlittle vegetation cover. The existence of large steep spillover81

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 7. Bear Island (Hammocks Beach). Large eastwardmigrating parabolic dunes have grown in elevation and volumesince the 1930’s. Dune forms are end products of the remobilizationof sand from the original forested ridges.lobes in the adjacent lagoon provides evidence for the landwardmigration of the sand dunes.Onslow BeachIn the vicinity of New River Inlet (Fig. 1) a submarineheadland forms a small seaward bulge in the coastline ofcentral Onslow Bay. This mid-compartment shoreline protrusionis produced by the Oligocene Silverdale Formation,an indurated moldic limestone and calcareous-cementedquartz sandstone unit. The Silverdale Formation crops out ator slightly below sea level in the mouth of the New Riverestuary. It occurs extensively on dredge spoil islands of theIntracoastal Waterway behind Topsail Island and OnslowBeach, and forms a series of bathymetric ridges on the innershelf on either side of New River Inlet (Crowson, 1980;Riggs et a11995; Cleary et al 1996). Crowson mapped theseprominent submarine rock features as a series of ridges thatoccur seaward of the lower shoreface, have steep landwardfacingscarps with smooth surfaces that dip gently away fromthe beach, and have up to 5 m of relief above the surroundingravinement surface. The ridges rise locally to about 5 mbelow MSL, higher than the elevation of the lower shoreface,which is high enough to cause refraction of storm waves andcurrents and possibly affect the patterns of erosion and depositionon the adjacent beaches.The ridges are oriented at acute angles to the beach andintersect the shoreface on Topsail Island and Onslow Beach(Fig. 8). Core drilling by Cleary and Hosier (1987) demonstratedthat the rock ridges :ontinue under Onslow Beach andinto the back- barrier estuarine system. Similar limestoneridges pass beneath Topsail Island and into the back-barrierestuarine system (Fig. 9) (Clark et al., 1986) where the rockstructures appear to be related to the occurrence and orientationof a Pleistocene bay barrier system.The Oligocene submarine headland appears to subdividethese two barriers into coastal compartments that have differentorientations and shoreface dynamics. The northern segmentof Onslow Beach is characterized by a cuspateshoreline geometry with wide beaches, a recurved accretionarybeach ridge system, a nearly continuous high foreduneridge, and shoreline accretion rates that average 2 mi yrFigure 8. Map showing the outcrop pattern of discontinuous rock scarps on shoreface influenced by Onslow/New River submarineheadland. Cartoon illustrates the location of the lowstand channel cut by the New River and the shoreline area where the ridgesintersect the barriers (After Riggs et al. 1995).82

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 9. Shore-parallel cross-section along Onslow Beach based on continuous split-spoon cores.The two rock ridges rise to within 5 m below sea level and pass beneath the island. A variety of channel and estuarine units characterizethe section recovered (after Cleary unpublished data and Riggs et al. 1995).(Cleary and Hoiser, 1987). The ridges front a narrow marshfilled lagoon and are covered with mature maritime forestindicating old and stable topography (fig. 10A). Toward thecentral portion of Onslow Beach, the lagoons along thenorthern and southern segments narrow (Fig. 10B), and thebarrier is perched on top of the limestone comprising theheadland (Riggs et al 1995; Cleary et al 1996). At the narrowestwidth the limestone lies within 3 meters of the surfaceof the fringing backbarrier marsh (Cleary and Hosier1987).The southern segment of the barrier is characterized by anarrow beach strewn with gravel and a discontinuous dunesystem composed of isolated "haystack" dunes with numerouswashover terraces extending into the marsh. The structureof the dune field is largely a result of the damage causedby the numerous manuvers and operations of the U.S.Marine Corps. Staging and landing operations involvingMarines and heavy equipment including tanks and large "airboats"have been carried out for decades. Current erosionrates approach 6 m/yr.Active bioerosion of rock scarps represent a majorsource and supply of "new sediment" to the adjacentbeaches. Abundant gravel, up to boulder-size grains, isderived from the rock scarps and lower shoreface. and deliveredto the beach during storms where it is rapidly brokendown to sand-sized grains in the surf zone (Crowson 1980;Cleary and Hosier 1987; Riggs et a11995; Cleary et aI1996).The highest erosion rates characterize the area immediatelyupdrift of New River Inlet. The southernmost portionof the barrier is currently undergoing very rapid eosion (Fig.11). Changes in the symmetry and volume of sedimentstored in the fronting ebb delta have accelerated the erosion.The erosion scenario is complicated due to the large numberof factors involved. The slow southward migration of theinlet coupled with the dredging activities since the mid1960's are major contributors to the rapid recession of theshoreline at the southern end of the island. Since 1960 therehas been a dramatic change in the offset pattern of the shouldersbordering the inlet. Currently the Onslow Beach shoulderis eroding rapidly while the Topsail Island shoulder is83

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 11. Map depicting the erosion rates along OnslowBeach (after Benton et al. 1993). Short term rates are muchhigher along the southern segment. Modification of New RiverInlet plays a key role in the erosion updrift on Onslow Beach.accerting slowly along a one and a half kilometer segmentdowndrift of the inlet (Cleary 1994)Topsail IslandFigure 10. Onslow BeachA. Oblique aerial view looking southwest from Brown’s Inlet.The marsh filled lagoon narrows to the south where the limestoneridges rise to within 3 m of the marsh surface. Accretionarydune ridges are evident in photograph.B. Oblique aerial view looking northeast from New River Inlet.The lagoon widens to the south towards New River Estuary.Pleistocene estuarine fill underlies much of the southern portionof the barrier. This area has undergone considerable recessionduring the past 30 years. Overwash is the dominantprocess involved in the barrier’s translation.Topsail Island is the second longest barrier island withinthe Onslow Bay section of North Carolina. The island is borderedby the New River Inlet to the North and New Topsaillnl,etto the south (Fig. 1 ). The island is approximately40 km long and averages approximately 280 m in width. Thenortheast- southwest barrier orientation exposes the island tofrequent winter storms.Studies by the US Army Corps of Engineers have shownthat, between 1856 and 1933, the northern half of the islandwas eroding at an average rate of 0.40 m/yr while the southernportion had alternating sections of accretion and erosion.Data from the period 1933 to 1980 indicate a slight increasein the erosion rate (0.70 m/yr) for the northern half, while thesouthern segment was characterized by sections of bothaccretion and erosion.Topsail Island is situated in a severe or chronic overwashzone (Fig. 12 A-E). Storms during the period 1944 to1962 and in the late 1980's were particularly devastating tothe island. Hurricane Hazel (1954) and the Ash Wednesdaystorm (1962) caused significant damage. Hurricane Hazeldestroyed 210 out of 230 buildings and generated a 2.9 mabove MSL flood level on an island whose average elevation84

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 12. Northern Topsail IslandA. Oblique aerial view (1974) looking north. Notice the lack of developmentand the narrow but intact dune field. Old washover terraces aremarked by shrub line.B. North view (1986) toward New River Inlet. Dune line is narrow, discontinuousand consists of scraped material. Washovers date from recent winterstorms. Inlet influenced accretion zone is at top of photo.C. North view of North Topsail (1994). The photo shows the controversialrelocated road positioned very close to the barrier’s landward margin.Bridges and culverts were constructed at low spots. This area is extremelylow and hazardous and will continue to be overtopped during storm events.D. Dune line pictured in 11B was severely eroded during New Year’s DayStorm 1987. Evacuation was difficult as surge crossed the road.E. Erosion of dune line prompted local officials to relocate road landward.This area is now located on the intertidal beach.85

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 13. Surf City. Surf City is located along the south centralportion of Topsail Island. The town is sited within aformer inlet zone (Stumpy Inlet, 18&19th c.). Finger canalsdredged in the 1960’s are sited on the former flood delta. Theentire area is low and fronted by a relatively narrow dune field.The area is prone to overtopping.is 2.7 m MSL. Hazel also removed 650,000 m3 of sand fromthe beaches at Topsail Beach and Surf City. Some of thissand was transported across the island toward the sound andmarsh in the form of washover fans. The grasslands and dunefields rest upon washover fan and terrace sediments. Thecrenulate border of the shrubs marks the landward edge ofthe overwash fans/ terraces (Fig. 12A) .The dune system along most of the island is generally asingle foredune often scarped and in some places discontinuous(Fig. 12A&B). Some areas do have massive or multipledunes, such as the small one kilometer segment downdrift ofNew River Inlet area and portions of areas in the southernsection (NC State Route 210 to Paradise Pier).Three inlets have affected the morphology and sedimentationalong Topsail Island since 1800. These are New River,Stump and New Topsail Inlets. Stump Inlet opened andclosed in the mid-1800's. The extensive vegetated flood tidalFigure 14. Aerial Photographs of New River.A. 1938 the inlet was narrow and choked with sediment. Theretention capacity of the ebb delta was limited due to thesmall tidal prism.B. 1986 photograph illustrates the enlarged throat and ebbdelta. Since maintenance dredging began in the early 1960’s,erosion has accelerated on Onslow Beach while accretion hasoccurred along a 1.5 km stretch of Topsail Island. Note offset,compared to “A”.delta of this inlet is now the site of Surf City. This area is lowand prone to overtopping (Fig. 13).New River Inlet which forms the northern boundary ofTopsail Island fronts the largest coastal plain estuary in theOnslow Bay compartment (Fig. 1 ). The position )f the inletis controlled by the location of the ancestral channel of theriver system. Cores, seismic data and 'he distribution of outcropson the innershelf indicate the paleochannel is incisedinto the Silverdale Formation. As a result of this incision, theshallow inlet las migrated within a 3.0 km zone alongOnslow 3each and Topsail Island.The hydrodynamics of this inlet were changed :considerablyby the dredging of Atlantic Intracoastal Waterway(AIWW) and the channels connecting the 3stuary with theopen ocean. The earliest photographs [1938) and charts indicatethe inlet and the main channels were clogged due to86

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 15. Aerial photograph (1986) of NewRiver area. The dune ridge complex on TopsailIsland began to develop in the early 1960’s.The road landward of the multi-unit structuresrepresents the 1960 shoreline. Accretion isrelated to the welding of swash bars andimpoundment within the natural fillet on thedowndrift side of the inlet. Ebb surge is likelyto breach the Onslow Beach shoulder promptinga change in the erosion pattern.Figure 16. Aerial photographs depicting morphological changes in the ebb delta and a portion of an ebb delta breaching cycle.Symmetry changes are determined by channel orientation and position (A-D). Erosion or accretion commences as breakwater effectof ebb delta is altered. Breaching of the ebb delta bypasses large quantities of sand to Topsail Island. The stage is set for such anevent in “D”.87

WILLIAM J. CLEARY AND ORRIN H. PILKEYreduced tidal flow. In 1940 a 3.7 km long navigation channelwas dredged connecting the Waterway with the inlet. Theearly 1960's marked the advent of sidecast dredging of thethroat and outer bar channel for navigation purposes. Followingdredging operations, the once small ebb tidal deltaincreased in areal extent from approximately 140,000 m 2 to700,000 m 2 . Due to the increased tidal prism, the volume ofsand retained in the ebb delta increased by almost 50 percent(Fig 14 A&B).Continuous southerly migration coupled with the dredgingof the throat and outer bar channel has produced an ebbdelta that is highly skewed toward Topsail Beach. Inresponse, the Topsail Island shoulder has prograded 40 msince 1959. Accretion associated with the inlet extends alonga 1.5 km zone and represents the only segment along thenorthern end of the barrier experiencing progradation (Fig.15). Due to the repositioning of the ebb and flood channelsacross the asymmetric ebb delta, there has been significantchanges in the seaward offset pattern of Topsail Island. All ofthe multi-unit dwellings along the northern end of the islandare sited seaward of the 1960 shoreline. A breach of updriftOnslow Beach across the narrow spit will result in repositioningof the ebb delta and a concommitant recession of theformer accretion zone (Cleary and Hosier 1987; Cleary1994; Cleary 1996).The large semi-circular pond on the northern end of theisland marks the former location of the southern shoulder ofthe inlet in the early 1900's. The inlet is likely to re-occupythis location if the present trend continues.New Topsail Inlet (Fig. 1) separates Topsail Island to thenortheast and Lea Island to the southwest. Historic coastal.charts and maps indicate this inlet existed as early as 1738.Since 1738, New Topsail Inlet has steadily migrated to thesouthwest, a distance of approximately 10.0 km. During theperiod 1856-1963 the inlet migrated 1830 m to the southwestat an average rate of 19.2 m/yr. Migration rates of 35 m/yrhave characterized the inlet over the last few years (1963-1994).Inspections of controlled aerial photographs from 1938-90 suggest the inlet cross-section has been asymmetrical,with the gorge positioned close to the Lea Island shoulderduring the majority of the period. As is commonly the casewith inlets in this region, the orientation of the main ebbchannel across the ebb tidal delta platforn changes on acyclical basis, dictating the patterns of erosion/accretion onthe adjacent shoreline (Fig. 16 A-D).Extensive beach front development on the southern endof Topsail Island began in the early 1950's. The cottages andmotels which date from this period were constructed on theprimary dune which paralleled the recurved portion of thesouthwesterly extending spit. Between 1950 and 1975, TopsailIsland's southern most 1500 m assumed a pronouncedbulbous shape. This shape historically has been a by-productof: 1 ) the reorientation of the ebb channel across the ebbtidal platform which afforded protection to the adjacentshoreline, and 2) the location of swash bars welding onto theTopsail Island spit (Cleary 1994). As New Topsail Inletmigrated, the bulbous portion also reformed to the southwestin accordance with the position of the inlet. This resulted in arealignment of the trailing shoreline {Fig. 17).The chronic erosion which currently characterizes thisarea {Fig. 18), stems predominantly from the recession ofthe primary recurved dune line as the inlet has migrated.Also, reorientation of the main ebb channel to the east-northeasthas caused subsequent erosion of a portion of the updriftshoreline. Erosion of oceanfront lots associated with NewTopsail Inlet migration and spit elongation has been acceleratedby the occurrence of nor'easters and hurricanes. Figure19 depicts the changes which took place in response to theseprocesses over a 10 year period. Shoreline recession in thisregion amounted to over 110 m during the 10 year period{1972-1982). The erosion rates have decreased during thepast decade.A recent study by the US Army Corps of Engineersshows the predominant direction of sand transport is to thenorth; between 55 and 60% of the total drift moving in anortheasterly direction. These data are in contrast to findingsof earlier studies. Also, it is difficult to resolve these dataFigure 17. Trailing barrier realignment. Cartoon illustrates erosion of the updrift barrier shoreline is a consequence of inlet migrationand associated planform changes.88

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 18. Erosion at Topsail Beach.A. December 1974 photograph. Development began in the 1960’s. Canals were dredges in 1969. Sea Vista Hotel (arrow) was completedin 1970 and was fronted by 130 m of uplands. Nylon bags were emplaced in 1972. Marsh islands (open arrow) representingformer locations of the inlet are seen in the lagoon.B. December 1978. The remaining single dune ridge was eroded by late 1978. Beach scarping followed.C. December 1984. High water line has recessed over 130 m. Sand bags were emplaced to halt erosion of the hotel.D. December 1984. Many of the homes located to the south of the Hotel were badly damaged due to erosion associated with shorelinerealignment and storms. Shown are homes built along recurved dune ridges. Note truncations of ridges.E. December 1984. Winter storms accelerated the erosion along much of south Topsail Island.F. April 1986. North view of realigned shoreline with scarped grassland. No dune field exists. Sea Vista Hotel is marked by blackarrow. A number of structures along this section were destroyed or relocated. The southern 5 km of the beach has been replenishedseveral times with sand from the adjacent flood delta. Erosion and overtopping is likely to continue in this area.89

WILLIAM J. CLEARY AND ORRIN H. PILKEYwith the fact that the inlet has migrated to the southwest overhistorical times. Localized drift reversal may account for thetrends but it is unlikely. Regardless of the sand transportdirection, the inlet is retaining large amounts of sand, with alarge portion being retained on the inner shoals {flood tidaldelta). The rapid shoaling in the sound side channels connectingthe Atlantic Intracoastal Waterway {AIWW) and theopen ocean is a consequence of the southwesterly movementof the flood tidal delta as the inlet migrates. Recent efforts tomaintain these channels with a side casting dredge have metwith little success.Lea IslandLea Island is the shortest island in the chain betweenCape Lookout and Cape Fear. It is approximately 2.5 kmlong and averages less than 200 m in width. Less than 8 % ofthe island has been in continuous existence for 50 years.Old Topsail Inlet, the island's southern boundary is veryshallow and is nearly closed. The lagoon, landward of thisinlet, is choked with reworked sand shoals associated withthe migration of the inlet. Data indicate that New TopsailInlet is 'pirating' a portion of the tidal prism of Old TopsailInlet. Much of Lea Island has been overtopped by majorstorms which have approached the area. As a result, overwashrelated physiography is dominant. The few large duneswhich once existed were found on the north part of the islandnear New Topsail Inlet.Coke IslandCoke Island, located immediately to the south of LeaIsland, is an undeveloped 4.5 km long, 180 m wide, overwash-dominatedbarrier. Since 1938, the island has lost over900 m of its shoreline due to the southerly migration of OldTopsail Inlet. The southern end of the barrier has remainedstable due to the relative stability of Rich Inlet.Storms have had a significant impact on the island producinga wide array of washover-related features and theopening of a short-lived inlet in 1959. This breach occurredabout 2 km north of Rich Inlet in a zone where the 18th centurySidbury Inlet was located. This area is marked by a linearmarsh island.Figure 19. Shoreline erosion updrift of New Topsail Inlet. Cartoon depicts shoreline positions in 1972 and 1981, inlet positions in1856 and migration rates. Maximum erosion rates were recorded at the 1972 position of the shoreline bulge, north of the fingercanals. Erosion is a result of planform adjustment (modified in part after USACE 1990 and Cleary 1994).90

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 20. Figure Eight Island and adjacent inlets.A. Rich Inlet, a stable inlet forms the northern border of FigureEight Island.B. Mason’s Inlet a small migrating system borders FigureEight Island to the south. Note the straightened shorelineupdrift of the inlet.Figure Eight IslandFigure 21. Rich Inlet Erosion.A. Ebb channel is shore-normal and flanked by wide floodchannel on Figure Eight shoulder. Encroachment of marginalflood channel onto south shoulder is prompted by deflection ofebb channel.B. Welding and migration of attached bars produced temporaryerosion in lee of sandbar. Sand packet eventually movesinto estuary and accretion commences. Erosion rates were ashigh as 2 m/day for six weeks time period in mid 1984. Area isnow fronted by a 200 m wide intertidal beach.Figure Eight Island is a narrow 6.5 km long island separatedfrom Coke Island by Rich's Inlet and from WrightsvilleBeach/Shell Island by Mason's Inlet (Fig. 20 A&B). Theisland exhibits two distinct physiographic sections. Thenorthern half of the island is narrow, yet possesses a high,continuous, basically single, forested dune ridge. TowardRich's Inlet, the island is offset seaward from Coke Island.The offset consists of a series of parallel dune ridges whichundergo erosion or accretion as the ebb tidal shoals of Rich'sInlet change (Fig. 20 A). Rich's Inlet has shown little tendencyto migrate, however, the cyclical re- orientation of theebb channel can produce very rapid erosion on adjacentshorelines (Fig. 21 ).Several 'marsh islands' are evident in the -lagoon behindFigure Eight Island (Fig. 20 B). These islands are characteristicallynarrow linear areas of higher elevation with the longaxis of the island parallel.to the seaward barrier island. Theseislands form landward of an inlet where flood tidal deltasands overtop the marsh. The higher ground is occupied byless salt tolerant plant species, including various shrubs andtrees. As an inlet migrates and/or closes, a chain of islands ispreserved within the marsh. Thus, these islands can be usedas indicators of the location of historic inlets in areas wherelagoons are infilled with marsh. Several good examples ofmarsh islands are found in the lagoon behind Figure EightIsland.The southern section of Figure Eight Island exhibits agenerally low, washover, inlet-influenced shoreline (Fig.91

WILLIAM J. CLEARY AND ORRIN H. PILKEY20B). A large recurved foredune marks the historic northernlimit of Mason’s Inlet. Sequential aerial photographs showthat the inlet has migrated more than 100 m since 1938.Bwfore construction of homes began along this section ofthe island in 1970, sand dredged from the sound side of theisland was deposited on the berm. Erosion along the southernhalf of the island was inconsequential until Mason’s Inletre-initiated a rapid migration to the south. Similar to eventsat Topsail Island, migration of the ebb channel of Mason’sInlet removed protecting bars of the ebb delta and exposedthe southernmost section of the island to erosion. Despite thepositioning of large sandbags to form a protective seawalland subsequent nourishment of the intertidal beach, erosioncontinues to threaten homes. Since the island is privatelyowned, the landowners themselves, not the Federal Government,are responsible for re-nourishment. Several renourishmentprojects have attempted to mitigate the chronic erosion.An additional phase is planned for late 1996.Wrightsville Beach and Shell IslandWrightsville Beach is a 7.3 km long developed barrierisland located east of Wilmington (Fig. 1 ). Because of itsproximity to the city of Wilmington, it was one of the firstbarrier islands in North Carolina to be developed as a resort.Bath houses and summer cottages built in the 1860's wereserviced by a trolley line that was completed in 1889 (Fig. 22A-C). This trolley ran a distance of 11 km from Wilmingtonacross the adjacent sound.Along much of the length of the island, one can seeexamples of man's interference with natural shoreline processes.Compilation of data from aerial photographs, cores,and historical charts show that all of the island rests on inletfill. Moore's Inlet, now closed (in the vicinity of the HolidayInn), was the major inlet in the area during the past century(Fig. 23 A&B). As late as 1920, an inlet was located in thevicinity of Mercer's Pier. Today, much of the marsh north ofthe pier rests on tidal delta sands.Masonboro Inlet (Fig. 1) to the south became a prominentinlet when Moore's Inlet began to close naturally in thelate 1940'5. It too, has influenced lagoon sedimentation andhas migrated over a distance of more than 2 km.Early photographs (1915-1920) show that the northernportions of Wrightsville Beach had large elevated dunes anda wide island profile. To the south the island Nas very narrowand low. In order to create more 91evated land, WaynickBoulevard, the road parallel :0 Banks Channel, was builtover tidal marsh in the 1930'5 (Fig. 24).Erosion on Wrightsville Beach is not a new problem.From the earliest attempts at building along the oceanfront,erosion problems have existed (Fig. 25). For example,between 1923 and 1939, more than two dozen concrete andtimber groins were emplaced along the shoreline in anattempt to halt erosion. The first attempt at replenishing thesand lost to erosion occurred in 1939, when 535,000 m3 ofsand were pumped onto the beach (US Army Corps of Engineers,1982).Between 1944 and 1965, four major hurricanes (includingHurricane Hazel, 1954) and a number of winter nor'eastersresulted in significant shorefront erosion. In 1965, theWrightsville Beach Erosion Control and Hurricane ProtectionProject was constructed along 4515 m of ocean shorelinewhich extended north from the Masonboro Inlet jetty(Fig. 26 A&B) to the town's northern limit.Additional sand was pumped onto the shore to closeMoore's Inlet, located 450 m north of the town (Fig. 23A&B). In all, a total of 2,280,000 m3 of sand was placed onWrightsville Beach. Subsequently, the town annexed the 762m section north of its original corporate limits whichincluded Moore's Inlet.Between 1938 and 1965, Moore's Inlet migrated along a1.5 km section of Wrightsville Beach and adjacent ShellIsland. Historic aerial photographs, maps, and charts showthis inlet affected the shape of the adjacent barrier islandbeaches by producing a convex shoreline protuberanceimmediately adjacent to the inlet (Fig. 23 & 26 A&B). Thisbulge is common along inlet influenced shorelines wheresand packets in the form of swash bars from the protectiveebb tidal delta are welding onto adjacent beaches. The endresult is a shoreline which curves seaward.Following the artifical closure of Moore's Inlet (1965),the building line and roads along the new northern corporatelimits were extended and basically paralleled the pre-closurecurved shoreline. Much of the erosion along the restorednorthern part of Wrightsville Beach stems from the relictconvex shape of the restored shoreline (Fig. 27).Evidence for rapid erosion along the newly annexedportion of Wrightsville Beach fronting Moore's Inlet wasobvious by the late 1960's. This recession necessitated theplacement of an additional 1 ,070,000 m3 of sand on thenorthern one-half of the beach. By the middle 1970's, homesand structures along the northern flanks of the bulge werefronted by bulkheads and walls of protective rip-rap (Fig. 28A&B) .Additional restoration in 1980 and 1981 placed1,380,000 m3 of fill along the northern 2450 m of theproject, temporarily reversing the shoreline retreat.In 1986, an additional 670,000 m3 Of sand was placedon the beach. US Army Corps of Engineers estimates thatthe convex shape of the shoreline accelerates the annual erosion(99,392 m3 of the fill by 31.5% (Jarrett, 1977; US ArmyCorps of Engineers, 1982). Figure 27 illustrates this erosionscenario at Wrightsville Beach where the continued restorationof the beach is aimed at protecting the structures along aseaward offset of the natural building line.Wrightsville Beach is one of the most-replenishedbeaches (i.e. large. federally-funded replenishment projects)on the U.S. East Coast (Pilkey and Clayton, 1987; 1989), andhas been funded under the widest variety of of federal autho-92

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 22. Historic Photographs ofWrightsville Beach.A. Early Banks Channel trestle.Trolley cars carried visitors toWrightsville Beach (ca. 1920). Thelarge hotel at the right is the Oceanic,a famous landmark in theearly days of development of thebeach. The original timbers of thetrestle were driven in the 1899.The structure connected the Hammocks(Harbor Island) andWrightsville Beach (D.H. Barnettcollection).B. Oceanic Hotel (ca. 1914). Nearsite of Wings (Newells) and Station#1. Originally constructed as theHotel Trymore in 1905, the Oceanicoffered its guests such amenitiesas a bowling alley, a spaciousballroom and well-appointedaccommodations.C. Station #7 (Lumina). Northview (ca. 1925) along old electrictrolley line. Note spacing of cottages,hotels and bath houses evenat this early date. Waynick Blvd. isyet to be constructed. (D.H. Barnettcollection).93

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 23. Aerial photographs of Moore’s Inlet, scale bar = 1,667 ft. Dot represents the location of the Holiday Inn.A. 1939. Photograph shows evidence of recent constriction of inlet. The main channel located on the Shell Island updrift shoulderis flanked by a wide shallow flood channel on the Wrightsville beach margin. Note curvature of dune lines on adjacent barriers.B. 1989. Erosion along this section of the beach is attributed to the closure of the Moore’s Inlet in 1965. Bulkheads, seawalls andrevetments that fronted the homes in this area in the 1970’s and 80’s are now buried by the sand from the replenishment projects.rizations of any beach in the U.S. (Pilkey and Clayton,1987). These include 1 ) Flood Control; 2) Emergency; 3)Flood Control and Navigation; and 4) Mitigation of theEffects of that Navigation Project. Major replenishmentshave been carried out at approximately four-year intervalssince 1965 (Table 1 ). each of which involved the placementof approximately 1 x1 06 m3 of material dredged from thebackbarrier lagoon and portions of Masonboro Inlet (Fig. 29A&B). Numerous engineering studies have investigated variousaspects of the Wrightsville Beach nearshore systemant}7- ...ts predicted response to jetty construction and beachreplenishment (e.g. Sager and Seabergh, 1977; Winton et a/.,1981; U.S. Army Corps ofEngineers, 1982). Over the pastseveral years, we have collected extensive geologic and geophysicaldata off Wrightsville Beach. Nearly 300 km of 3.5kHz subbottom profile and 100 kHz analog sidescan-sonardata have been obtained as part of previous projects fundedby the NOAA National Undersea Research Program. Thegeophysical data were obtained in June 1992, and cover abroad area of the shoreface and inner continental shelf. Asuite of over 100 short (-2 m) percussion cores, vibracores,surface sediment samples and diver observations wasobtained between ,1991-1992. In March 1994, a fully georeferenced(:t 3m accuracy), high-resolution digital sidescan-94

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 24. Southview of Wrightsville Beach (ca. 1920). Southviewof Wrightsville Beach (ca. 1920) in vicinity of Station #1(Wings). Note the narrowness of the southern portion of thebarrier. Cottages and hotels line the island from ocean tosoundside. The tracks for the early trolley were laid within thedune swales. The old road bed of the railroad underliesLumina Ave. Banks Channel is seen on the right side of thephoto. Dredge material from this channel provided fill for theconstruction of Waynick Blvd. and the initial USACE restorationof the beach in 1965. (D.H. Barnett collection).Figure 25. Historic photograph of Wrightsville Beach (ca.1930). Historic photograph illustrating location of groins andbulkhead along the area south of Moore’s Inlet. Severe erosionis evident. Note bulkhead on soundside. Large Hotel isthe Oceanic which burned in 1935.sonar mosaic of the shoreface and inner shelf off WrightsvilleBeach was produced as part of a joint cooperativeresearch program with Duke University-U.S. GeologicalSurvey. Another digital sidescan-sonar survey was conductedin early August1995, followed by the collection ofadditional vibracores and samples located on the basis of thetwo sidescan-sonar mosaics.The morphology of the Wrightsville Beach shoreface isdominated by shore-normal rippled scour depressions (agenetic term used by Cacchione et al., [1984] to describesimilar features in other shelf environments). The depressionsdevelop just outside the fair-weather surf zone at 3-4 mwater depth, and extend to the base of the shoreface at about1 Om depth. On the sidescan-sonar mosaic, the rippled scourdepressions are defined by areas of high acoustic reflectivity.The depressions are floored with very coarse shell hash andquartz gravel, and on the upper shoreface are scoured up to 1m below the surrounding areas of fine sand. Long, straightcrested,symmetric megaripples floor the depressions. Thedepressions terminate and the shore-normal morphologicfabric becomes shore-oblique at the base of the shoreface,due to a series of east- to northeast- trending relict ridgeswith 1-2 m of relief.The more numerous rippled scour depressions along thesouthern part of the Wrightsville Beach shoreface may be theresult of increased bedrock control, as evidenced by largerareas of rock outcrops on the shoreface and inner shelf. Onthe 1992 sidescan- sonar data, small areas (20-50 m2) of outcroppingrock appear to be exposed above the fine sandbetween some of the depressions. Onshore water-weII logsand ground-penetrating radar surveys on the island have alsoidentified Tertiary limestone units in the near subsurface inthe same area where rippled scour depressions are abundantoffshore. In addition, the gross morphology of the shorefaceand inner shelf did not change over a 21-month periodbetween the 1992 and 1994 sidescan-sonar surveys.The surficial sediment distribution in the 1994 and 1995surveys is nearly identical to a sedimentary facies map presentedby Thieler et al. (1995) based on analog sidescansonarand surface sample data collected in June 1992. Theseobservations suggest that the locations of some rippled scourdepressions on the shoreface may be controlled by bedrocktopography; they may also be relatively permanent features.The shoreface sediment cover off Wrightsville Beach isa patchy veneer blanketing low-relief, ancient units. Themodern sediment, including the contribution from the nearbyreplenished beach, averages about 30 cm in thickness. Theprimary underlying units are a Plio- Pleistocene arenaceouslimestone, unconsolidated Oligocene silt, and Quaternaryfiuvial channels. In addition to our geophysical data, Snyderet al. (1994) have also identified the seismic signature andjistribution of these units across the southern Onslow Bayshelf .Petrographic analysis of surface sediment samples onthe shoreface and inner shelf indicate several jistinct, localsediment source areas. The sources include the three ancientunits, in addition to the modern beach. For example, thereare a number of locations in the study area where limestoneoutcrops are present, some of which are productive hardgroundhabitats. Bioerosion of the outcrops produces a residualsediment ranging in size from gravels to lime mud. Thisresidual fraction is mixed with outcrop- associated, relatively95

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 26. Wrightsville Beach Photographs.A. North view of Wrightsville Beach and the bulge in the mid barrier shoreline. The north jetty was constructed in 1965 and thesouth rock jetty in 1981. Oceanic Pier is located north of the weir jetty. Much of Wrightsville Beach is fill material.B. 1965 oblique aerial photograph showing construction of the weir jetty. The structure acted as terminal groin for the fill placed onthe updrift beach. Oceanic pier (Crystal) is located in the foreground (USACE).Figure 27. Shoreline changes associated with closure of Moore’s Inlet in 1965. During the 1940’s the inlet produced a curvature inthe adjacent shorelines. Concurrent with closure, the building line was established that mimicked the curvature. Recessionoccurred as the offshore portion (ebb delta) of the bulge as well as the onshore region eroded. During the 1970’s several housesrequired protection in the form of seawalls and bulkheads. Some structures were relocated. The hard structures can be observedafter storms. During the 1980’s and 1990’s sand from replenishment projects have afforded some protection to this area.96

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 28. Erosion along the shoreline bulge.A. Seawall shown was emplaced in late 1970’s. Rip-rap fronts a bulkhead to the north. Area is located immediately north of HolidayInn.B. North view of Wrightsville Beach in 1980. Artificial dune is severely eroded. Old groins are exposed.fresh invertebrate fragments, including small corals and shellmaterial.Most of the vibracores exhibit a sharp, erosional contact(the Holocene ravinement surface) between the active sedimentcover and an underlying Oligocene silt unit, indicatingperiodic erosion and bypassing of material onto the shoreface.The similar mineralogy of the ancient unit and immediatelyadjacent modern sediments indicates the Oligoceneunit is contributing glauconite-rich silt and very fine sand tothe inner shelf. Backbarrier sequences deposited during theHolocene have infilled relict inlet and tidal creek channelsincised into the Pleistocene and Tertiary units. Three radiocarbondates for in situ oysters from these channels providean age assignment of 8-10 ky, with age increasing with depthand distance offshore. These deposits, some of which arevisible on the sidescan-sonar mosaics, are eroded andreworked on the mid- to lower shoreface during stormevents, providing a minor source of material for the overlyingshoreface sediment cover.Sediment for beach replenishment projects was dredgedfrom the backbarrier lagoon and portions of MasonboroInlet. Of the total 7. 7x1 06 m3 of sediment emplaced, atleast 60 percent (4.4x106 m3) is composed of a macroscopicallyidentifiable, gray quartz sand with abundant, grayblack-stainedrecent oyster shells ( Crassostrea virginiGa)(volume and source area data from U.S. Army Corps ofEngineers, 1982). The replenishment sediment is nearlyidentical to the native beach and shelf material in terms of its97

WILLIAM J. CLEARY AND ORRIN H. PILKEYgrain size distribution, carbonate content, and physicalattributes of the carbonate fraction (shell abundance, particlesizes, etG.) (U.S. Army Corps of Engineers, 1982; confirmedby our unpublished data). The major difference between thebeachfill and the native sediment is that the faunal componentof the beachfill material is characterized by black- andgray-stained oysters, while the native beach and shelf sedimentcontains oxidized, brown- and orange-stained shells ofmarine genera (e.g., Anadara, Donax).Some of the sediment from the early beach replenishmentprojects can be found on the shoreface and inner shelf.Pearson and Riggs (1981) first noted the occurrence of thisreplenishment sand, which is identifiable on the basis of itsgray color, black-stained shell material, and high oyster shellcontent. The replenishment sediment is visibly distinct fromthe "ancient" oyster-bearing sediments described above. Specifically,the physical condition of the oyster shells in the twosuites is quite different. The ancient oysters are typically ableached, whitish color, and are fairly fragile. Whenexhumed by shoreface erosion, the shells quickly break up,and are oxidized to a light, brown-orange. This contrastssharply with the well- blackened, durable, modern specimensfrom the replenishment projects. We are presentlyusing this unique sediment tracer to identify decadal-scalesediment dispersal patterns on this shoreface.Masonboro IslandMasonboro Island extends along 13 km of the shorelinebetween Wrightsville Beach and Carolina Beach Extension.Masonboro Inlet separates the island from WrightsvilleBeach; Carolina Beach Inlet separates it from CarolinaBeach to the south (Fig. 30 A&B). Masonboro Island wascontinuous with Carolina Beach until 1952 when CarolinaBeach Inlet was opened.Masonboro Island is typical of the low islands formingthe eastern limb of the Cape Fear Foreland. This is the onlyopen ocean barrier island in North Carolina that is currentlyin a state of migration. Characteristically, the island possesseslow relief and borders a narrow (ca. 2 km) partiallymarsh-infilled lagoon. The subaerial portion of the islandvaries from 50 to 600 m in width; most of the island averages175 m. Since the island is situated along the major stormtracks, it receives significant impacts from hurricanes andnor'easters, and is frequently overtopped.The morphologic character of the barrier has changeddramatically over the past 20 years. Photographs from 1938show that vegetated dunes formed a continous dune line98

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 29. Wrightsville Beach.Eroded “dune”. The 1975 photograph shows the nature of theoriginal USACE fill which consisted of poorly sorted oystershells, shell gravels, fine sand and mud. The material wasderived from Banks Channel.North view of Hurricane Protection Project (1995). Theproject extended along approximately three miles of the southernportion of the beach.Figure 30. Masonboro Island aerial photographs.North view of Masonboro Island (1981). Storm deposits characterizethe barrier. Since the 1980’s the island has recededrapidly. The 1996 shoreline is located within the maritimeforest in the center of the photo.North view (1994) of Masonboro Island from Carolina BeachInlet. The southern segment of the barrier is low and erodingrapidly due to the opening of Carolina Beach Inlet in 1952.Overwash during storms is the driving variable that is mostimportant in island migration. Currently the area shown is asingle continuous washover terrace.along the island length except near Masonboro Inlet. Thestorm period between 1954 and 1962 produced extensiveoverwash fans and terraces. Following the 1962 "AshWednesday" extra-tropical storm, nearly 70 % of the islandexhibited washover topography.The dune system that had been obliterated during the1960's began to recover and by 1980, 80% of the barriershoreline was characterized by scattered redeveloping dunes.A series of storms in the mid 1980's coupled with the modifictionof Masonboro Inlet has dramatically altered the characterof the island. During -the 1987 New Year's Day storm,80% of the barrier was overtopped forming extensive fansthat extended into the adjacent estuary. Currently, about 18%of the shoreline exhibits a dune line.Downdrift of Masonboro Inlet, dune progradation is evident,the first since 1945. The progradation followed the constructionof the south jetty in 1981. Except for this smallshoreline segment the remainder of the barrier is rolling-overon itself fairly rapidly. Short term erosion rates are 4-5 m/yr.The increased washover suseptibility and high erosion ratesare attributable to the maintenance of Carolina Beach Inletand the dual jetties at Masonboro Inlet. The reduced by-passingat the island's southern end and the nearly completeretention of sediment at the northern boundary has drasticallyreduced the sediment supply. The combined effect ofthese two sediment sinks has produced a defecit of approximately250,000 m/y.If present trends continue, it is highly likely that the99

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 31. Cabbage Inlet. 1994 aerial photograph. CabbageInlet opened in 1761 during the most intense storm to makelandfall in the area. Closure occurred in 1780. The extensivetidal marsh behind the island represents the vegetated formerfloor tidal delta. Note the forested hammocks which representthe shoulders of the former inlet. Currently washovers extendbeyond the forest.southern two-thirds of the island will resemble AssateagueIsland, Maryland in the next several decades. The high erosionrate, lack of dunes, and chronic overwash will lead torapid translation of the barrier toward the mainland. It islikely the island will migrate the equivalent of several islandwidths by the year 2025 and have a markedly different shape.An extensive area of marsh exists in the lagoon approximatelyin the center of the island. This marsh marks the positionof Cabbage Inlet, a major inlet which was open duringthe 1700's (Fig. 31 ). Inlets have not been long lived alongthe island, probably due to the small tidal prism associatedwith the narrow lagoon. Vibracores recovered from thelagoon indicates the thickness of the Holocene fill rangesfrom 3 m on the interfluves to more than 10 m in the deeperparts of the backfilled incised valleys. Sands or muddy sandsdominate the sequences refelecting the influence of ephemeralinlets and oceanic overwash. Core data suggest thatinter-tidal environments have characterized the lagoon sincethe initial flooding began 5,000 years ago.Carolina Beach, Carolina Beach Extension andKure BeachThe barrier island chain of the Cape Lookout to CapeFear section of the North Carolina Coast Is interrupted atCarolina Beach. The marsh-filled estuary found north, andagain south, of Carolina Beach does not exist behind theCarolina-Kure Beach section of the shore (Fig. 32 A). Thisportion of the coast is characterized by a perched mainlandbeach. Elevations directly landward of the beach are 6 to 10m. Pleistocene-aged, erosion-resistant subsoils extend ontoFigure 32. Carolina Beach/Kure Beach aerial photographs.A. North view of Carolina Beach and southern terminus of thelagoon. Carolina Beach Inlet is seen in foreground. MasonboroIsland and Carolina Beach were contiguous until 1952when the inlet was opened. Erosion rates increased markedlyfor several years following the breach. The lake in the lowerright portion represents a former estuary that was connectedto the ocean at the turn of the century. The connection wasinfilled creating a hazardous situation during times of heavyrainfall or overtopping. Flooding is typical and roads areimpassable.B. Rip-rap along Carolina Beach Extension. The rock rubblewas emplaced in 1970-73 in an attempt to prevent further erosiondowndrift of Carolina Beach Inlet. The 2000 ft long structurefronts a re-entrant in the shoreline.the beach at several locations. This shoreline has had a colorfulhistory of shoreline stabilization attempts similar to thoseundertaken at Wrightsville Beach. Various generations ofgroins, beach berm construction and beach nourishment areevident along the shoreline. The projects undertaken sincedevelopment began (early 1900's) have proved to be shortterm;the erosion of the mainland beach has persisted. Erosionrates of up to 1 m/yr have been measured.A major beach fill borrow site has been targeted on the100

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFort Fisher Beach and East BeachFigure 33. Fort Fisher aerial photograph. North view (1987).The beach along the Fort Fisher/Kure Beach section isperched atop Pleistocene units comprised of two lithologies:Coquina and a friable humate rich sandstone. Coquina cropsout on the beach front of the townhouses in center of photo.The underlying geology plays a critical role in the local erosion.hardbottom dominated shoreface off Carolina Beach. Thesite represents an anastomosed channel complex of theancestral Cape Fear River. The Pleistocene channels are estimatedto contain in excess of 15 million cubic meters ofsand, a sufficient volume to satisfy the local needs for thenext decade.Carolina Beach Extension marks the northern end of thebarrier island physiography. This section of the shoreline issediment starved. Carolina Beach Inlet intercepts considerablequantities of sand moved alongshore by the longshorecurrent. As a result, a re-entrant has formed south of the inlet(Fig. 32 B). Overwash is the dominant process along thissection of beach. Dunes have little time to recover beforewashover events erode their edges. Despite a severe shoalingproblem, proposals to close Carolina Beach Inlet have notbeen favored because recreational and fishing boatsanchored at Carolina Beach would be required to enter andexit the ocean at Masonboro Inlet, 13 km distant from CarolinaBeach 1nlet.In this area, an extensive eroding subaerial headlandintersects the coastal zone without a barrier island- estuarinesystem (Fig.1 & 33). The shoreline segment consists of awave-cut platform incised into Oligocene through Pleistoceneunits (Fig. 34 A&B) of the mainland peninsula, with athin beach perched on top of the irregular geometry of thePleistocene units (DuBar et al., 1974; Moorefield, 1978;Meisburger, 1979; Cleary and Hoiser, 1979; Snyder et al.,1994, Riggs et al., 1995; Cleary et al., 1996).Erosion resistant, lithified and crossbedded coquinasandstone forms a headland in the shoreline north of FortFisher (Fig. 34 A). Friable humate and iron- cemented Pleistocenesandstone forms a 2 m high wave-cut cliff and terracethat fronts the shoreline immediately south of the headlandand seaward of the Civil War Fort Fisher. South of FortFisher is a nonheadland segment characterized by a channeldominated,valley-fill shoreface underlain by 10 m of muddyestuarine sediments (Swain and Cleary, 1992) . The shapeand evolution of the three different coastal compartmentsaround Fort Fisher is clearly related to the presence andlithology of the outcropping and underlying Pleistocene geologicframework.Moorefield (1978) mapped beach outcrops of Pleistocenecoquina north of Fort Fisher and their seaward extensionson the inner shelf. Our ongoing studies clearly showthat coquina and its associated lithologies form a series ofwidespread, irregular bathymetrically high hardbottom featureswith > 3 m of relief. This karstic mosaic includes oneextensive hardbottom area known as Sheephead Rock thatlies in 9 m of water with pedestal-Iike hardbottom featuresrising to within 2.5 m of the ocean surface (Fig. 35 A&B).Diver observations and cores suggest that the sediment coveris both patchy and very thin across much of this region andin many areas is totally lacking. The extensive series ofcoquina outcrops on the inner shelf act as barriers that couldsignificantly affect the refraction of wave energy, as well asthe movement of sand across this shoreface. Sand from boththe rapidly eroding beach at Fort Fisher and littoral drift, istransported seaward of the outcrops during storms and preventedfrom returning to the beach during subsequent lowenergy periods. The result of this process is a net sedimentdeficiency in which the rapidly retreating bluff shoreline isconsuming the historic fort.Although the coast from Kure Beach to Fort Fisher istechnically part of the mainland, the location of Cape FearRiver on the back side of the area effectively creates a landformsimilar to that of a barrier island. Most of the developmenthere is more than 7 m above mean sea-Ievel, on stable,vegetated areas and can be considered safe. This safety isindicated by the hurricane record for Kure Beach as comparedto that of other beach resorts in the area. HurricaneHazel, for example, severely damaged or destroyed hundreds101

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 34. Coquina, sandstone outcrops and perched beach.A. Coquina units crop out on the beach and underlie much ofthe headland beaches. The unit extends onto the shorefaceand plays a crucial role in cross-shore transport duringstorms.B. Organic rich sandstone. Dark brown humic stained sandstonesoverlie the coquina units. The sandstones and thepaleo pine forest stumps are frequently exposed after storms.of buildings at Carolina and Wrightsville Beaches, but only80 buildings at Kure Beach. In most of the other 1950'sstorms, Kure Beach survived with minor damages, and HurricaneDiane (1955) is said to have helped build up the duneline. Even if an area is considered safe, shoreline developmentis still at risk. It is not uncommon for sand to bewashed into the streets of the first block facing the beach.Some structures, such as piers, must be placed on the beach.Owners should plan for damage and replacement. (KureBeach fishing pier has been destroyed eleven times.)The beach south of Fort Fisher to the Brunswick Countyline resumes the barrier island chain. The East Beach complex(Fig. 36 A&B) extending south from Fort Fisher is a 9km complex spit which connects the Holocene sediments ofthe Cape Fear Foreland and the older Pleistocene headlandsection (Carolina- Kure-Fort Fisher).Use of off-road vehicles on the beach south of FortFisher has caused changes in the physiography and vegetation.Vehicle use is somewhat restricted; vegetation removalhas opened sands on the subaerial portion of the spit to remobilization.As a result, the dune system covers a largerportion of the spit when, compared to East Beach furthersouth. The overall vegetation cover has been decreased byvehicles. The potential for erosive, ocean-to-sound washoversis more likely on the beach where vehicle use is uncontrolled(Hosier and Eaton, 1980).The East Beach section of the shoreline has thinned tothe point where the subaerial portion of the island ranges inwidth from 40 to 300 m and inlet breaches and washoversare common (Cleary and Hosier, 1979). The foredunes areweakly developed and form a discontinuous ridge. Shorelinerecession has been calculated to occur at a maximum rate ofapproximately 5 m/yr immediately south of Fort Fisher.Migratory inlets have been a common feature of EastBeach during the last 150 years. Extensive areas of marshbuilt upon flood tidal deltas fill the lagoon behinQ EastBeach. Currently New Inlet occurs in this section (Figs. 36B&37). Historical records and charts show the original NewInlet opened in 1761 during a severe hurricane. The breachoccurred in a low and narrow region known as the 'haulover'.It is very likely that one or more historic inlets preceded NewInlet.The inlet channel which formed in 1761 deepened andremained essentially stable until 1839 when it began to shoaland migrate in a southerly direction. In 1854, attempts weremade to close the breach which led to an accumulation ofsediments in the Cape Fear River Channel to the west. In1881, a dam {'The Rocks') was completed which effectivelycut off tidal exchange between the Cape Fear River and theestuary riverward {landward) of East Beach {Fig. 37).Between 1895 and 1960, a cycle of inlet opening, migrationand closure was repeated three times along a 2 km section ofbeach. Recurved spit features and a network of tidal creeksin the lagoon landward of East Beach indicate the historic102

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 35.A. Sheephead Rock Fathometer Traces (Coquina Outcrops). Representative bathymetric profile across a large high relief outcrop ofPleistocene age coquina, located offshore 3.7 km due south of Fort Fisher. Coquina is composed of coarse shell-hash and 10 cm pelecypodvalves. Well indurated Pliocene barnacle rich hardbottom surfaces underlie this calcarenite sequence. Depressions at variousdepths are partially sediment filled.B. Sidescan-Sonar Mosaic of Coquina Outcrops (Sheephead Rock). Sonograph shows area of high relief (>4 m) coquina (calcarenite)outcrops that rise to within 2m of the sea surface. (A) A series of ridges delineate well indurated gravel-rich coquina units, indicatedby high backscatter (dark areas). (B) “Fishscale patterns” denote Patchy sands and gravels covering rocky outcrops. (C)Areas of fine sand are indicated by low backscatter (white areas).location of these former inlets.Construction of the dam not only produced a uniquetype of estuarine system, it also set the stage for subsequenterosion events along the updrift shoreline segment at FortFisher. Prior to inlet closure in 1881 , a large asymmetric ebbshoal containing a minumum of 30 million m3 fronted theFort Fisher shoreline. The highly skewed ebb delta acted as anatural breakwater and protected the updrift shoreline segmentagainst direct wave attack. Closure of the inletprompted the collapse of the shoal as the tidal prism of theinlet was drastically reduced. The remobilized sedimentinfilled the former throat section and fed the newly developedspit (Fig. 37).In the early part of this century, major sections of thecoquina that crops out on the beach along the Fort Fisherarea were removed for road building and construction materials.Closure of New Inlet and the removal of the coquinaultimately led to a shoreline recession exceeding 17 m/yr103

WILLIAM J. CLEARY AND ORRIN H. PILKEYstate, the project was initiated in 1995. Plans called for a3,050 ft. rock revetment with a crestal elevations of 10-16.5ft., a base width of 70 ft. and an armored toe consisting of 5ton interlocking STA-POD units (USACE 1993). The projectwas completed in the spring of 1996 at a cost of approximately$4 million (Fig. 39).The debate over the Fort Fisher seawall was an importantone, made more difficult by the fact that there is no roomfor compromise on the shoreline armoring issue. The variancegranted to construct this wall was the first one given outon non federal land. It has already been cited by homeownersanxious for shoreline armoring as reason for another variance.A number of other historic sites on barrier islands havefallen victim to the sea including Fort Wagner (the locationof the events in the movie Glory) on Morris Island nearCharleston. If we are to prohibit shoreline armoring for thegoal of preserving beaches for future generations, can exceptionsbe made?The state of North Carolina will be monitoring theimpact of the seawall on the adjacent beaches. There is concernthat the structure will enhance the headland effect of thecoquina outcrops and accelerate the erosion of the downdriftbeaches and possibly the updrift segment as well. Manyother environmental concerns have been raised and arebeyond the scope of this brief discussion of the site.Bald Head IslandFigure 36. East Beach aerial photographs.A. North view of spit attached to Fort Fisher headland. Offroad vehicle traffic has impacted much of the dune field.B. South view of East Beach toward New Inlet. The inlet’smigration rate is a function of its position within various compartmentsof the Zeke’s Island estuary. The Cape Fear Riveris in the upper right portion of the photo.between 1926 and 1931 (Beach Erosion Board, 1931). Followingthe hurricanes of 1954 and 1955, several small groinsand rubble from storm related destruction were placed in theembayment immediately south of the coquina outcrops. In1970 a rock revetment consisting of limestone from CastleHayne was emplaced. Since the mid 1970's a variety of constructionrubble has been added to the site. Erosion rates ofapproximately 3 m/y were recorded in the 1980's along theshoreline segment fronting the fort (USACE 1993).In order to mitigate the rapid erosion, a Beach ErosionControl Project was authorized in 1976 to protect the CivilWar earthen mound fortifications. The historic fort wasreduced to approximately 50 % of its original extent at thetime of the authorization. After obtaining a variance from theBald Head Island is an exclusive developed barrierisland located at the mouth of the Cape Fear River Estuary.Bald Head Island is a 9 km long, forested, beach-ridge barrier.Bald Head, and 3 smaller islands separated from it bytidal marsh, are part of once more extensive Holoceneregressive sequence that has since been drowned by risingsea-Ievel. Collectively, the sequence is part of the offshoreshoals that extend onto the continental shelf from Cape Fear.The origin of the Cape Fear Foreland as well as the othertwo Capes in North Carolina (Capes Hatteras and Lookout)have been related to ocean current eddies (Dolan and Ferm,1968) and erosional remnants of Pleistocene deltas (Hoytand Henry, 1971 ). Data from the shelf shoals suggest theCapes 1lay be quite old and related to subtle structural leatures(Blackwelder, et al, 1982).Regardless of their antiquity or origin, the present daymorphology of the three islands that form the foreland complexdate from approximately 4,500 years B.P. when sea-Ievel rise is thought to have decelerated. At this time shorelineprogradation commenced. The progradational phasemay have lasted 2500 years or longer, the exact length isspeculative for it is difficult to determine without detailedstratigraphy and radiocarbon dates on the age of the beachridges. Since the last progradational episode, rising sea levelhas drowned the low swale areas, all of which are nowinfilled with tidal marsh and crossed by large tidal creeks.104

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCFigure 37. Aerial photograph of the Zeke’s Island Estuary and the “Rocks”. The dam (Rocks) was a major engineering feat of thenineteenth century. Upon completion in 1881, it stretched for one mile between the headland and Zeke’s Island. The southern sectionwas completed in the subsequent five years. The former New Inlet which opened in 1761 was a major inlet and was used by theBlockade Runners during the Civil War and was one of the main reasons for the construction of the Fort. Rapid infilling of thenewly formed estuary occurred as the offshore shoals collapsed and the shoreline adjusted. Closure ultimately played a major rolein the erosion of the shoreline fronting Fort Fisher. Currently (1996) the inlet is located at the southern end of the estuary (arrow).The geometric arrangements of the historic multipledune sets reflect the change in the pattern of the shoalsimmediately offshore both at the eastern and western ends ofthe island. The eastern end is characterized by truncated forestedbeach ridges with sets of smaller multiple dunes orientedperpendicular to the ridge complex. During majorstorms the majority Df the eastern shoreline is overtoppedresulting in the formation of large overwash terraces whichextend into the marsh.SUMMARYThe Onslow Bay section of the North Carolina coastJetween Cape Lookout and Cape Fear possesses a lariety ofbarrier islands and a short section of 11ainland. Islands varyfrom more than 40 km to less han 3 km in length with islandwidths similarily lariable. In addition to the underlying geologicramework, the processes of oceanic overwash; inletormation, migration, and closure and eolian transport ~Ilaffect these islands with differing patterns and ntensities. Asa result, each island, like individual people, takes on a different'personality' based on the relative influence of each ofthese processes. In a similar fashion, each of these islandsare different with regard to the potential for development.Although ndividual homesites on any island can be relativelysafe" or relatively dangerous, some valid island wide Jeneralizationscan be made about developmerJt ;uitability. Thelow transgressive islands such as ropsaillsland are much lesssuitable for development han the sand rich regressive islandssuch as Bogue 3anks. The heavy forests of Bogue Bank provide;ome high wind protection while treeless Carolina3each provides none. By analyzing the existing norphologyof the barrier islands, some of the history of an island can beinferred and a reliable prediction of future changes can bemade.REFERENCES CITEDBeach Erosion Board. 1931. Fort Fisher, N.C., House DocumentNo.204, 72nd Congress, 1 st Session, Report of Beach ErosionBoard of the United States Army Corps of Engineers.Blackwelder, B. W., Maclntyre, I. G. and Pilkey, D. H. 1982. Geologyof Continental Shelf, Onslow Bay, North Carolina, asRevealed by Submarine Outcrops: Bull. Am. Assoc. Pet. Geol.,v.66, pp. 44-56.Cacchione, D.A., Drake, D.E., and Glenn, S., 1984. Rippled scourdepressions on the inner continental shelf off central California.105

WILLIAM J. CLEARY AND ORRIN H. PILKEYFigure 38. New Inlet closure. Cartoon illustrates changes in the shoreline and ebb delta due to inlet closure. The civil works projectproduced a unique type of estuary and led to the collapse of the large ebb delta (80% reduction) as the tidal prism was drasticallyreduced. The long term consequences led to erosion of the Fort Fisher shoreline as the ebb delta reorganized and its breakwatereffect was eliminated.Figure 39. Fort Fisher Seawall. Aerial Photograph (1996). The 3,050 ft long revetment completed in early 1996, was a controversialissue which required a variance and special permit. Crestal elevations range from 10 to 16.5 ft. The 70 foot wide base is fronted byhuge 5 ton interlocking pods. The structure faired reasonably well during Hurricane Bertha and suffered minor damage duringHurricane Fran.106

ENVIRONMENTAL COASTAL GEOLOGY: CAPE LOOKOUT TO CAPE FEAR, NCJournal of Sedimentary Petrology, 54: 1280-1291.Clark, P., Cleary, W.J., and Laws, R.A. 1986. ALate Pleistocenebay-barrier system: Topsail Sound, North Carolina: GeologicalSociety of America. Abstracts with Programs, 18(3), 21Cleary, W.J., 1994, New Topsail Inlet, North Carolina. Migrationand Barrier Realignment: Consequences for Beach Restorationand Erosion Control Projects. Union Geographique Internationale,Commission Sur de l'Environment Cotier C. Institute deGeographique p. 116-130.Cleary, W. J. and Hosier, P. E., 1979. Geomorphology, WashoverHistory, and Inlet Zonation: Cape Lookout, N.C., to BirdIsland, N.C. In: Leatherman, S. P. (ed), Barrier Islands: AcademicPress, New York, N. Y. p. 237-271.Cleary, W.J., and Hosier, P.E., 1987, Onslow Beach, N.C.: Morphologyand Stratigraphy, Proc., Coastal Sediments '87, NewOrleans, p. 1760-1775.Crowson, R.A., 1980. Nearshore rock exposures and their relationshipto modern shelf sedimentation, . Onslow Bay, North Carolina.Unpub. M.S. Thesis, Dept. of Geology, East CarolinaUniv., Greenville, 128 p.Dolan, R. and Ferm, J. C., 1968. Crescentic Landforms Along theAtlantic Coast of the United States: Science,v. 159, pp. 627-629.DuBar, J.R., Johnson, H.S., Thorn, B.G., and Hatchell, W.O., 1974.Neogene stratigraphyand morphology, south flank of the CapeFear Arch, North and South Carolina, in Oaks, R.Q., andDuBar, J.R., (eds.), Post Miocene Stratigraphy; Central andSouthern Atlantic Coastal plain: Logan, Utah, Utah State UniversityPress, pp. 139-173.Fisher, J. J., 1962. Geomorphic Expression of Former Inlets: Alongthe Outer Banks of North Carolina: Thesis, Univ. North Carolina,120 p.Hayes, M.O., 1994, The Georgia Bight barrier system: in Davis(ed), Geology of Holocene Barrier Islands systems., SpringerVerlag, Chapter 7, p. 233-305.Heron, S. D. Jr., Moslow, T. F., Berelson, W. M., Herbert, J. R.,Steele, G. A., and Susman, K. R., 1984, Holocene Sedimentationof a Wave- Dominated Barrier Island Shoreline: CapeLookout, North Carolina: Marine Geology, v. 60, pp. 413-434.Hine, A. C. and Snyder, S. W., 1985. Coastal Lithosome Preservation:Evidence From the Shoreface and Inner Continental ShelfOff Bogue Banks, North Carolina: Marine Geology, v. 63, pp.307-330.Hosier, P. E. and Cleary, W. J., 1977. Cyclic Geomorphic Patternsof Washover on a Barrier Island in Southeastern North Carolina,Environ. Geol. v.2: pp. 23-31.Hosier, P. E. and Eaton, T. E. 1980. The impact of vehicles on duneand grassland vegetation on a southeastern North Carolina barrierbeach. J. App. Ecol. v. 17, pp. 173-182.Hoyt, J. H. and Henry, V. J., 1971. Origin of Capes and ShoalsAlong the Southeastern Coast of the United States: Geol. Sac.Am. Bull., v. 82, pp. 59-66.Jarrett, J. T., 1977. Sediment Budget Analysis, Wrightsville Beachto Kure Beach, N.C. In: Coastal Sediments '77. American Associationof Civil Engineers, New York, N. Y., pp. 986-1005.Meisburger, E.P. 1979. Reconnaissance geology of the inner continentalshelf, Cape Fear region, North Carolina: U. S. ArmyCorps of Engineers, Coastal Engineering Research Center;Techical Report, TP79-3, 135p.Moorefield, T.P. 1978. Geological processes and history of the FortFisher coastal area, North Carolina: Masters. Thesis, Departmentof Geology, East Carolina University, Greenville, NorthCarolina.Moslow, T. F. and Tye, R. S., 1985. Recognition and Characterizationof Holocene Tidal Inlet Sequences: Marine Geology, v. 63,pp. 129-151.Oertel, G. F., 1985. The Barrier Island System, In: G. F. Oertel andS. P. Leatherman (eds), Barrier Islands. Mar Geol., v63:pp. 1-18.Pearson, D.R., and Riggs, S.R., 1981, Realtionshipof surface sedimentson the lower forebeach and nearshore shelf to beachnourishment at Wrightsville Beach, North Carolina: Shore andBeach, v. 49, p. 26-31.Pilkey, O.H., and Clayton, T.D., 1987, Beach replenishment: Thenational solution? In Coastal Zone '87, New York, ASCE,p.1408-1419.Pilkey, O.H., and Clayton, T.D., 1989, Summary of beach replenishmentexperience on U.S. East Coast barrier islands: Journalof Coastal Research, v. 5, no.1, p. 147-159.Riggs, S.R., Cleary, W. J., and Snyder, S.W. 1995.. Influence ofinherited geologicframework on barrier shoreface morphologyand dynamics. Marine Geology, 126, 213-234.Sager, R.A., and Seabergh, W.C., {977, Physical Model Simulationof the Hydraulics of Masonboro Inlet, North Carolina: Vicksburg,Mississippi, U.S. Army Corps of Engineers, WaterwaysExperiment Station, GITI Report 15, 552 p.Snyder, S.W., Hoffman, C.W., and Riggs, S.R., 1994. Seismicstratigraphic framework of the inner continental shelf: MasonInlet to New Inlet, North Carolina. North Carolina GeologicalSurvey, Bulletin, 97, 61 p.Steele, G. A., 1980. Stratigraphy and Depositional History of BogueBanks, North Carolina: Thesis, Duke University, Durham, N.C.p. 201.Thieler, R.E., Brill, A.L., Cleary, W.J., Hobbs, C. H. III, and Gammisch,R.A. 1995. Geology of the Wrightsville Beach, NorthCarolina shoreface: Implications for the concept of shorefaceprofile of equilibrium. Marine Geology, 126, 271-287.Swain,K.W., and Cleary, W.J., 1992, Modification of a Coastal Plain/Bar Built Estuary, Southeastern North Carolina, Abstract withPrograms, Geologic Society of America, Southeastern section,Winston-Salem, N.C., v. 24, no 2, p. 69.U.S. Army Corps of Engineers, Wilmington District, 1982. FeasibilityReport and Environmental Assessment on Shore and HurricaneWave Protection, Wrightsville Beach, North Carolina,Wilmington, N.C.U.S. Army Corps of Engineers, Wilmington District, 1993. Phase IIGeneral Design Memorandum Supplement, Fort Fisher, NorthCarolina.Winton, T.C., Chou, I.B., Powell, G.M., and Crane, J.D., 1981,Analysis of Coastal Sediment Transport Processes fromWrightsville Beach to Fort Fisher, North Carolina: Vicksburg,Mississippi, U.S. Army Corps of Engineers, Waterways ExperimentStation, Miscellaneous Report. No. MR; 81-6, 205p.This article represents UNCW's Center for Marine ScienceResearch contribution #150.107

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 109 - 117APPENDICESAppendix 1. North Topsail Island 1986-87Appendix 2. Post Hurricane Bertha (7/18/96) Northern Topsail IslandAppendix 3. Post Hurricane Bertha (7/18/96) Northern Topsail IslandAppendix 4. Post Hurricane Bertha. Northern Topsail IslandAppendix 5. Post Hurricane Fran (9/15/96) Northern segments of Topsail IslandAppendix 6. Post Hurricane Fran (9/15/96) Surf CityAppendix 7. Post Hurricane Fran (9/16/96) Surf CityAppendix 8. Post Hurricane Fran (9/15/96) Masonboro Island109

Appendix 1. North Topsail Island 1986-87A. Bulldozed dunes were commonplace along the northern segment of the island. Note relative difference in elevation of berm androad. Peat is exposed near old pier.B. Bulldozers were employed to scrape sand from the lower beachface after the numerous winter storms that impacted the area inthe mid 1980’s.C. Bulldozers often exhumed peat and cedar stumps during excavations. Wave swash during high tide reached the base of thescraped dune line. Natural dunes did not rebuild.D. The surge associated with the New Years Day Storm of 1987 eroded much of the bulldozed dune line.110

Appendix 2. Post Hurricane Bertha (7/18/96) Northern Topsail IslandA. The 5-6 ft surge produced ocean to sound washovers. Lower floods and garages were most vulnerable.B. Washover fans extended into sound along much of the northern section of the island. One meter thick deposits were common onthe marshes and former grasslands.C. The low spots traversed by the relocated road were particularly prone to overwash and scour during overtopping. These samelocations were hard hit by Hurricane Fran.D. The washover events associated with Hurricane Bertha in some instances penetrated the marshes almost to the edge of theICWW.111

Appendix 3. Post Hurricane Bertha (7/18/96) Northern Topsail IslandA. View looking north. Low lying developed areas near bridges and culverts were severely damaged. Guardrail was damaged bylarge concrete slab transported from beach 25 m seaward.B. Sheets piles were emplaced along low areas crossed by the relocated road. Overwash sediments infilled the low regions and builtfans that extended into the open water areas.C. The narrow scarped dunes and portions of the grasslands along much of the northern segment of Topsail Island were eroded. Theformer road is exposed.D. Fragments of the old road bed could be found on the relocated road after the storm. In some instances the old road rests atoppeat and cedar stumps.The narrow scarped dunes and portions of the grasslands along much of the northern segment of TopsailIsland were eroded. The former road is exposed.E. Erosion of the uplands along this section set the stage for the destruction wrought by Hurricane Fran.F. All the fronting dunes have been eroded as well as the majority of the grassland. The lower floors of all the units in this photowere damaged. No storm protection was in place for the hurricane that was to follow.112

Appendix 4. Post Hurricane Bertha. Northern Topsail IslandA. Low spots beneath bridges acted as washover passes or sluices. Overwash penetrated from ocean to sound. Dark areas on beachrepresents garnet rich heavy mineral lag deposits. Some peat is exposed.B. Northern top of island. View to south from New River Inlet. The dune field downdrift of the inlet afforded protection for themulti-unit dwellings along the northern 1 km of the island.C. Several of the seaward dune ridges of the accretion complex were eroded during the storm. Overwash deposited material in theintervening swales as breaches in the dune ridge were opened.D. Oblique view south. See caption for “A”.E. Aerial view to south. Overwash is located in and around the Villa Capriani. Note the position of the old road and the relocatedhighway. Dune line is basically non-existent. The bridge to the mainland is located near the designation for the Pleistocene barrier.113

Appendix 5. Post Hurricane Fran (9/15/96) Northern segments of Topsail IslandA. Peat and relict cedar stump forests outcropped along much of the beach. The old road was built atop the peat and forest soil.B. Most of the “inlets” were low spots of former wetlands. Peat was found in most cases on the seaward side of the beach. Concretewas poured along the road bed to prevent future breaching and failure.C. Beach looking north. Recovery of the beach was not evident along this portion of the island. Arrows mark the locations of cedarstumps. Rock and large fragments of fossil oysters litter the upper beach.D. Stump filed in the right portion of photo is the same field pictured in “C”. Erosion during Fran removed all remaining dunes andburied or eroded grasslands in the region. Lava-like concrete is in place to protect road and hasten “closure”.114

Appendix 6. Post Hurricane Fran (9/15/96) Surf CityA. North view. Front row of homes were destroyed along a portion of Surf City. Poor construction provided the tools and projectilesfor the surge to further damage the landward row of homes. Notice orientation of pilings. Beach consists of a variety of gravels andboulders derived from the shoreface. Ridge and runnel systems were evident along this sector of Surf City.B. Homes once located on the seaward row were rafted off their pilings (arrow). Most if left intact, came to rest against the landwardrow of homes, resulting in major damage. Note the complete lack of a dune field.C. Multiple surge pulses associated with the storm produced a stacking of poorly constructed homes as they piled one against another.D. Some of the rafted homes remain remarkably intact. Some rested upon thick overwash deposits.E. & F. Establishing the setback line along much of Surf City will be the subject of much debate. The natural dunes are unlikely toredevelop to their former extent. Scraped dunes offer little protection and provide a false sense of security. Much of this area is a naturalhazard zone and once was the site of Stump(y) Inlet. It is likely to be impacted by every major storm event.115

Appendix 7. Post Hurricane Fran (9/16/96) Surf CityA. Site is south of new high rise bridge. All of the lower floors and garages in this development were damaged by overwash. Washoverfans and terraces extend along the entire area into the marsh.B. Locations where mobile homes were sited did not fare well due to the 10-12 ft surge. A meter of more of sediment was depositedin this area. Many of the trailer clusters were located in the lowest and most vulnerable spots.C. Seaward view. Debris is from destroyed oceanfront homes. Much of the region fronting the mobile homes had a poorly developeddune.D. Overwash terrace. Many cars and other groundfloor items were “washed out” by the pulses of surge.E. A trailer jam on the seaward side of the road in Surf City. This site was a portion of an extensive overwash terrace.F. Debris consisting of pilings and concrete footings and framing caused considerable damage.116

Appendix 8. Post Hurricane Fran (9/15/96) Masonboro IslandA. View looking toward Wrightsville Beach and the Masonboro Jetty. The dune field developed in the fillet were destroyed for themost part. The remaining dune ridge was breached in several places. At these sites washover fans extended into the marsh.B. View to north, approximately 2.5 miles south of the jetty. A steep beach was characterized by a large ridge and deep runnel system.Along this section washover terraces extended well into the tidal marsh. No dunes remain and most grasslands are eroded aswell.C. View to north approximately two miles north of Carolina Beach Inlet. Terrace is extremely flat and overtopped by waves at hightide. Boulder size peat clasts are derived from the surf zone. Peat ages are less than 1.5 ka.D. Seaward view immediately south of Cabbage Inlet. Oligocene sandstones and Pleistocene mud along with organic rich units ofvarying age crop out on the steep beach. The sandstone forms a bulge in the Masonboro Island shoreline.E. The forested and shrub thickets that developed on the shoulders of the old inlet are now covered by extensive washover deposits.Recession of as much as several hundreds of feet has occurred in this area in the last several years.F. View north. Site is located approximately one mile south of “E” at Old Cabbage Inlet. Inlets which were reported to have openedin this area were low, broad and extremely flat terraces. Photography was taken at high tide. Elevations in this area should increasewith time as a berm rebuilds. It is highly unlikely that dunes will redevelop. The entire southern 9 km of the barrier is being translatedlandward via overwash.117